chore: pin Bun 1.4.4 #902

Why

Issue #898 bundles Bun, Node and Go bumps with a root-cause hunt for the openai-request-headers failures. The owner now wants CI and every local machine on one Bun right away: pin the latest stable Bun 1.4.4 and update local installs to match. Node and Go stay in #898, because the local Node 26 is a major jump that needs its own review.

Scope

• Pin Bun 1.4.4 in package.json (packageManager, @types/bun), sdks/vscode/package.json, flake.nix (Intel macOS baseline URL and hash), nix/README.md and script/toolchain.test.mjs.
• Update only the @types/bun and bun-types entries of bun.lock; a local bun install rewrites unrelated registry URLs.
• Out of scope: Node and Go pins (chore: upgrade the pinned toolchain to Bun 1.4.4, Node 24.22.1 and Go 1.27.3 #898), dependency bumps, and any SpecGit file (SpecGit stays local to each machine and is never tracked).

Approach

CI reads the Bun version from package.json through script/toolchain.mjs, so the pin moves CI and the Nix shell together. If the three openai-request-headers transport-body tests (seen failing on Bun 1.4.3) still fail on 1.4.4 in CI, fix their cause in this change instead of loosening them.

Acceptance

• node --test script/toolchain.test.mjs passes and bun install --frozen-lockfile accepts the lockfile.
• Typecheck, Unit Tests (linux) and E2E Tests (linux, windows) pass in CI on Bun 1.4.4.
• Machines on Bun 1.4.4 pass the push hook's Bun check.