test(server): stabilize JWKS validation tests

[grs]

Summary

Prevents intermittent failures from the jwks_validation tests.

Related Issue

No issue required: just a test fix

Changes

• Keep Wiremock servers alive for the full lifetime of JWKS cache tests, preventing pooled servers from being recycled prematurely.
• Replace real-time sleeps in the stale-key eviction test with explicit cache timestamp manipulation.
• Increase the test TTL window to prevent scheduler delays from crossing grace-period boundaries.
• Verify that the first unknown-kid lookup performs a JWKS refresh.
• Place subsequent unknown-kid lookups deterministically within the cooldown window.
• Preserve the existing assertions for JWKS refresh throttling, stale-key retention, and eventual eviction.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[pimlock]

/ok to test 31f5fcc