[medium] Fix stale cache after session key rotation in x402-client

[davedumto]

Summary

After a session key rotates, x402-client.ts continues to serve cached authorization results tied to the old key.

Requirements

• Invalidate relevant cache entries as part of session key rotation
• Add a test verifying post-rotation calls do not use stale cached authorization
• Add a debug log entry for cache invalidation on rotation
• Document the invalidation step in the x402 README section

[themancalledemma]

@themancalledemma has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi maintainer can I work on this issue?

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[drips-wave]

Congratulations, @themancalledemma! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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