Fix client-side Copilot token handling and add tests

[SteVen Batten (sbatten)]

This pull request addresses two critical client-side failures related to the Copilot token refresh mechanism, specifically during outage scenarios. The changes include:

• Enhanced the FetchedValue utility to manage failure retry windows and prioritize Retry-After handling.
• Migrated the completions token cache to utilize FetchedValue, implementing a five-second cooldown for ordinary failures.
• Propagated HTTP 429 responses and Retry-After metadata through the token minting process.
• Prevented alternating-error retry loops by removing error-message changes as triggers for token change events.
• Added comprehensive regression tests to ensure the reliability of the new behavior.

The implementation focuses on maintaining a compact codebase while effectively addressing the identified issues, with a total of 304 lines added, primarily for tests. All validation checks have passed successfully.

[Copilot]

Pull request overview

Improves Copilot token refresh resilience during outages and rate limiting.

Changes:

• Adds retry-window and invalidation support to FetchedValue.
• Propagates rate-limit metadata and applies bounded backoff.
• Migrates completion token caching and adds regression coverage.

Show a summary per file

File	Description
shared-fetch-utils/common/test/fetchedValue.spec.ts	Tests retry and invalidation behavior.
shared-fetch-utils/common/middleware/rateLimitBackoffMiddleware.ts	Exposes shared backoff constants and header parsing.
shared-fetch-utils/common/fetchedValue.ts	Adds failure cooldowns and generation-safe invalidation.
platform/authentication/vscode-node/copilotTokenManager.ts	Converts rate limits into bounded blocked-fetch errors.
platform/authentication/test/node/copilotToken.spec.ts	Tests HTTP 429 metadata propagation.
platform/authentication/test/node/authentication.spec.ts	Tests token-change event semantics.
platform/authentication/node/copilotTokenManager.ts	Parses and propagates retry metadata.
platform/authentication/common/copilotToken.ts	Extends token errors with retry timing.
platform/authentication/common/authentication.ts	Stops events caused only by changing errors.
completions-core/.../auth/test/copilotTokenManager.spec.ts	Tests completion token failure caching.
completions-core/.../auth/copilotTokenManager.ts	Migrates completion token caching to FetchedValue.

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• Files reviewed: 11/11 changed files
• Comments generated: 0
• Review effort level: Balanced