fix(gax): abort stalled REST fallback fetches

[akshatbaranwal]

Summary

• treat the second REST fallback stub argument as metadata headers and the third as call options
• wire deadline to an actual abort timer for REST fallback fetches
• make server-stream cancel() abort the underlying fetch instead of only the parser-owned controller
• add regression coverage for unary deadline abort, server-stream deadline abort, and server-stream cancel

Why

In REST fallback mode, server-streaming reads can silently stall after fetch dispatch but before the first response byte. Before this change:

• the generic timeout wrapper populated the third argument, but the fallback stub only read the second
• server-stream cancel() aborted the parser-owned controller, not the fetch-owned controller

That combination leaves stalled REST fallback fetches effectively unbounded and uncancellable.

Testing

• cd core/packages/gax && npm run compile
• cd core/packages/gax && npx mocha build/test/unit/grpc-fallback.js