fix: avoid forced Microsoft OAuth consent

[calvinmclean]

Microsoft authorization requests inherited prompt=consent from the generic provider, which can trigger an unnecessary "Approval required" flow even when tenant-wide admin consent already exists.

This regressed in #13, when provider-specific authorization parameters were replaced with unconditional oauth2 options.

• use prompt=select_account for Microsoft so users can choose another account without forcing consent
• retain prompt=consent only for Google's authorization endpoint, preserving its refresh-token behavior
• leave existing access_type=offline, requested scopes, and PKCE URL generation unchanged
• add focused coverage for Microsoft, Google, and providers that require no prompt

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/providers/generic.go	85.71%	1 Missing and 1 partial ⚠️

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[Copilot]

🟢 Approval recommended

The implementation matches the stated behavior and has focused regression coverage.

Pull request overview

Restricts Google-specific OAuth parameters to Google authorization endpoints, preventing forced Microsoft consent while preserving scopes and PKCE.

Changes:

• Detect Google endpoints by hostname before adding offline consent options.
• Add focused Microsoft, Google, generic-provider, and PKCE URL coverage.

File summaries

File	Description
pkg/providers/generic.go	Applies provider-aware authorization options.
pkg/providers/generic_test.go	Verifies generated authorization parameters.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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