Bun parity: preserve Connection headers after typed arrow erasure

Corpus location

• Position: 4129
• File: js/web/fetch/fetch-connection-header.test.ts
• First observed failure: SyntaxError: Unexpected token ':'. Expected ')' to end an argument list.

Required parity

• Correctly erase the typed async arrow signature (fetchOptions: RequestInit): Promise<Record<string, string>> => without damaging the surrounding JavaScript.
• Erase the local Record<string, string> annotation used by the captured-header object.
• Preserve explicit Connection values and their value casing while matching the header name case-insensitively.
• Provide Bun's default Connection: keep-alive when the caller does not supply the header.
• Preserve other request headers alongside Connection.
• Keep any header validation failures structured with stable operation/header context, causes, and useful stacks.

Completion gate

• Make the complete copied Bun corpus file pass.
• Add focused rewrite/runtime regressions and pin the file in the permanent HTTP/web mini-suite.
• Keep copied-corpus accounting and the project build green.

Parent tracker: #66

Implemented and pushed to main in 9a5d459c7dfd109ea1c83389a2e9e1abde15ec77.

• Bootstrap TypeScript erasure now removes the typed async arrow return signature and Record<string, string> local annotation without leaving invalid JavaScript.
• HTTP/1.1 fetch dispatch supplies Bun's default Connection: keep-alive only when the caller did not provide a value.
• Explicit Connection values retain their casing, header names remain case-insensitive, and accompanying headers are preserved.
• Added a focused rewrite/runtime regression and pinned the file in the permanent HTTP/web mini-suite.

Verification:

• js/web/fetch/fetch-connection-header.test.ts: 10 passed, 0 failed, 0 todo, 0 unsupported
• js/web/fetch/fetch-cyclic-reference.test.ts: 2 passed, 0 failed, 0 todo, 0 unsupported
• focused Connection-header regression: passed
• permanent HTTP/web tail queue: passed
• zig build --cache-dir /private/tmp/home-zig-abi-cache: passed