fix: MOS token cache timezone comparison bug in TryGetCachedToken

[pratapladhani]

Summary

Fix MOS token cache timezone comparison bug and improve 401 error guidance. Fixes #277.

Problem

Two bugs in a365 publish that together cause persistent HTTP 401 errors on machines with timezones ahead of UTC:

Bug 1: DateTime timezone mismatch in TryGetCachedToken

DateTime.TryParse with a UTC "Z" suffix converts the timestamp to local time by default, then the comparison with DateTime.UtcNow uses raw tick values without timezone conversion. This makes expired tokens appear valid for extra hours equal to the UTC offset:

Timezone	Extra hours token appears "valid"
IST (+5:30)	~5.5 hours
JST (+9:00)	~9 hours
AEST (+10:00)	~10 hours

Bug 2: Cache file location mismatch in error guidance

The 401 troubleshooting message said "Delete: .mos-token-cache.json" without the full path. The cache lives at ~/.a365/mos-token-cache.json but users look in %LOCALAPPDATA%\Microsoft.Agents.A365.DevTools.Cli\ where other CLI config files are stored.

Changes

File	Change
MosTokenService.cs	Add CultureInfo.InvariantCulture + DateTimeStyles.AdjustToUniversal to DateTime.TryParse so parsed expiry stays in UTC
PublishCommand.cs	Show full cache path (~/.a365/mos-token-cache.json) in 401 troubleshooting message
MosTokenServiceCacheTests.cs	6 new regression tests covering cache validity, timezone parsing, buffer, and cross-environment isolation

Testing

• 20/20 MosTokenService tests pass (14 existing + 6 new)
• Build: 0 warnings, 0 errors
• No other DateTime.TryParse instances in the codebase need this fix (checked)
• Key regression test (AcquireTokenAsync_CachedTokenUtcZSuffix_ParsedAsUtcNotLocalTime) writes a token expired 3 hours ago in UTC — with the old code, IST machines would still treat it as valid for ~2.5 more hours

Code Review Checklist

• No "Kairo" keyword references
• Microsoft copyright headers on all files
• IDisposable properly disposed (test class implements IDisposable, restores cache file)
• Cross-platform compatible (no Windows-specific paths)
• No secrets or hardcoded values
• Tests use DisableParallelization pattern (shared cache file)

[Copilot]

Pull request overview

This PR fixes a critical timezone-related bug in MOS token caching that caused persistent HTTP 401 errors on machines with timezones ahead of UTC. The bug occurred because DateTime.TryParse was converting UTC timestamps to local time before comparison with DateTime.UtcNow, making expired tokens appear valid for extra hours equal to the UTC offset. The PR also improves the 401 error troubleshooting guidance by displaying the full cache file path.

Changes:

• Fixed DateTime timezone comparison bug in TryGetCachedToken by adding CultureInfo.InvariantCulture and DateTimeStyles.AdjustToUniversal to ensure parsed timestamps remain in UTC
• Updated 401 error message to show the full cache path (~/.a365/mos-token-cache.json) instead of just the filename
• Added 6 comprehensive regression tests for token cache validation, timezone parsing, expiry buffer, and environment isolation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
MosTokenService.cs	Fixed DateTime.TryParse to use DateTimeStyles.AdjustToUniversal, preventing timezone conversion that caused expired tokens to appear valid
PublishCommand.cs	Updated 401 error message to display full cache file path for better user guidance
MosTokenServiceCacheTests.cs	Added new test file with 6 regression tests covering cache validity, timezone parsing bug, 2-minute buffer, and cross-environment isolation