[Pure C] Harden OAuth token caching in the Zerobus Rust SDK

[teodordelibasic-db]

• Reproduce quoted expires_in values and failed proactive refreshes.
• Cache numeric and quoted positive lifetimes while rejecting invalid values.
• Continue serving a valid cached token when refresh fails until actual expiry.
• Add deterministic concurrency, invalidation, cancellation, and expiry tests.
• Update rust/NEXT_CHANGELOG.md and run the relevant Rust checks.