fix(make-pdf): Bun.which-based binary resolution for browse + pdftotext on Windows

Extends v1.24.0.0's Bun.which + GSTACK_*_BIN override pattern (introduced in
browse/src/claude-bin.ts via #1252) to the two other binary resolvers in the
codebase: make-pdf/src/browseClient.ts:resolveBrowseBin and
make-pdf/src/pdftotext.ts:resolvePdftotext.

Same Windows quirks (fs.accessSync(X_OK) degrades to existence-check; `which`
isn't available outside Git Bash; bun --compile --outfile X emits X.exe), same
Bun.which-based fix shape, same env override convention.

Changes:
  - GSTACK_BROWSE_BIN / GSTACK_PDFTOTEXT_BIN as the v1.24-aligned overrides;
    BROWSE_BIN / PDFTOTEXT_BIN remain as back-compat aliases.
  - Bun.which() replaces execFileSync('which', ...) for PATH lookup. Handles
    Windows PATHEXT natively; no more `where`-vs-`which` branch.
  - findExecutable(base) helper exported from each module, probes .exe/.cmd/.bat
    after the bare-path miss on win32. Linux/macOS behavior is bit-identical
    (isExecutable short-circuits before the win32 branch ever runs).
  - macCandidates renamed posixCandidates (always was — /opt/homebrew, /usr/local,
    /usr/bin). No Windows candidates added; Poppler installs scatter across
    Scoop/Chocolatey/portable zips and guessing causes false positives.
  - Error messages get a Windows install hint (scoop install poppler / oschwartz10612)
    and `setx` example for GSTACK_*_BIN.
  - Pre-existing test 'honors BROWSE_BIN when it points at a real executable'
    was hardcoded /bin/sh — made cross-platform via a REAL_EXE constant
    (cmd.exe on win32, /bin/sh on POSIX). Was a Windows-CI blocker on its own.

Coordination: PR #1094 (@BkashJEE) covered browseClient.ts independently with a
narrower scope; this PR's pdftotext + cross-platform tests + GSTACK_*_BIN naming
are additive. Either order of merge works.

Test plan:
  - bun test make-pdf/test/browseClient.test.ts make-pdf/test/pdftotext.test.ts
    on win32 — 29 pass, 0 fail (12 new assertions: findExecutable POSIX/win32/null,
    resolveBrowseBin GSTACK_BROWSE_BIN + BROWSE_BIN + precedence + quote-strip,
    same shape for resolvePdftotext + Windows install hint in error message).
  - POSIX branch unchanged — fs.accessSync(X_OK) on Linux/macOS short-circuits
    before any win32 logic runs, matching the v1.24 claude-bin.ts pattern.

make-pdf/test/pdftotext.test.ts

@@ -8,7 +8,8 @@
 
 import { describe, expect, test } from "bun:test";
 
-import { normalize, copyPasteGate } from "../src/pdftotext";
+import * as path from "node:path";
+import { normalize, copyPasteGate, findExecutable, resolvePdftotext, PdftotextUnavailableError } from "../src/pdftotext";
 
 describe("normalize", () => {
   test("strips trailing spaces", () => {
@@ -104,3 +105,103 @@ describe("copyPasteGate — assertion logic", () => {
     expect(Math.abs(expectedBreaks - tooManyBreaksNormalized)).toBeLessThanOrEqual(4);
   });
 });
+
+// ─── Binary resolution (v1.24-aligned) ──────────────────────────
+
+const REAL_EXE: string =
+  process.platform === "win32"
+    ? path.join(process.env.SystemRoot ?? "C:\\Windows", "System32", "cmd.exe")
+    : "/bin/sh";
+
+function withEnv<T>(overrides: Record<string, string | undefined>, fn: () => T): T {
+  const saved: Record<string, string | undefined> = {};
+  for (const k of Object.keys(overrides)) saved[k] = process.env[k];
+  for (const [k, v] of Object.entries(overrides)) {
+    if (v === undefined) delete process.env[k];
+    else process.env[k] = v;
+  }
+  try {
+    return fn();
+  } finally {
+    for (const [k, v] of Object.entries(saved)) {
+      if (v === undefined) delete process.env[k];
+      else process.env[k] = v;
+    }
+  }
+}
+
+describe("findExecutable (pdftotext.ts)", () => {
+  test("returns the bare path on POSIX when it's executable", () => {
+    if (process.platform === "win32") return;
+    expect(findExecutable("/bin/sh")).toBe("/bin/sh");
+  });
+
+  test("on win32, probes .exe / .cmd / .bat after the bare-path miss", () => {
+    if (process.platform !== "win32") return;
+    const base = path.join(process.env.SystemRoot ?? "C:\\Windows", "System32", "cmd");
+    expect(findExecutable(base)).toBe(base + ".exe");
+  });
+
+  test("returns null when no extension matches", () => {
+    expect(findExecutable("/nonexistent/path/to/nothing")).toBeNull();
+  });
+});
+
+describe("resolvePdftotext (override resolution, v1.24-aligned)", () => {
+  test("honors GSTACK_PDFTOTEXT_BIN when it points at a real executable", () => {
+    // We can't fake a real pdftotext, but we can fake "any executable" to
+    // exercise the override-resolution path. describeBinary will mark flavor
+    // as "unknown" since cmd.exe / /bin/sh don't respond to -v like pdftotext;
+    // the test asserts on the bin-path resolution, not the version probe.
+    const info = withEnv({ GSTACK_PDFTOTEXT_BIN: REAL_EXE }, () => resolvePdftotext());
+    expect(info.bin).toBe(REAL_EXE);
+  });
+
+  test("honors PDFTOTEXT_BIN as a back-compat alias", () => {
+    const info = withEnv(
+      { GSTACK_PDFTOTEXT_BIN: undefined, PDFTOTEXT_BIN: REAL_EXE },
+      () => resolvePdftotext(),
+    );
+    expect(info.bin).toBe(REAL_EXE);
+  });
+
+  test("GSTACK_PDFTOTEXT_BIN takes precedence over PDFTOTEXT_BIN", () => {
+    const info = withEnv(
+      { GSTACK_PDFTOTEXT_BIN: REAL_EXE, PDFTOTEXT_BIN: "/nonexistent/legacy" },
+      () => resolvePdftotext(),
+    );
+    expect(info.bin).toBe(REAL_EXE);
+  });
+
+  test("strips wrapping double quotes from override values", () => {
+    const info = withEnv({ GSTACK_PDFTOTEXT_BIN: `"${REAL_EXE}"` }, () => resolvePdftotext());
+    expect(info.bin).toBe(REAL_EXE);
+  });
+
+  test("error message includes Windows install hint and GSTACK_PDFTOTEXT_BIN", () => {
+    let thrown: unknown = null;
+    try {
+      withEnv(
+        {
+          GSTACK_PDFTOTEXT_BIN: "/nonexistent/gstack-pdftotext",
+          PDFTOTEXT_BIN: "/nonexistent/pdftotext",
+          PATH: "",
+          Path: "",
+        },
+        () => resolvePdftotext(),
+      );
+    } catch (err) {
+      thrown = err;
+    }
+    // If the test box has a real pdftotext on disk, resolution succeeds
+    // (POSIX candidates) — that's fine; the assertion is gated on whether
+    // it threw. On Windows-CI without poppler, it throws.
+    if (thrown) {
+      expect(thrown).toBeInstanceOf(PdftotextUnavailableError);
+      expect((thrown as Error).message).toContain("pdftotext not found");
+      expect((thrown as Error).message).toContain("GSTACK_PDFTOTEXT_BIN");
+      expect((thrown as Error).message).toContain("Windows");
+      expect((thrown as Error).message).toContain("scoop install poppler");
+    }
+  });
+});