fix(webhooks): isolate half-open probe state per consumer URL

[Olasunkanmi975]

##closed #510

Summary

Fixes the in-memory webhook circuit breaker to enforce exactly one half-open probe per consumer URL, matching the Redis implementation and preventing probe interference across endpoints.

Changes

• Isolated half-open probe tracking on a per-URL basis.
• Ensured probe admission is atomic and limited to one in-flight probe per consumer URL.
• Reset probe state correctly after successful and failed probe attempts.
• Added TSDoc documenting the half-open probe semantics and invariants.
• Added concurrency tests verifying per-URL isolation and parity with the Redis implementation.

Testing

Covered:

• Concurrent probes for multiple URLs.
• Exactly one probe admitted per URL.
• Repeated probes for the same URL.
• Probe state reset after success.
• Probe state reset after failure.
• In-memory behavior matches Redis semantics.

Validation

npm test

Security

This change preserves the exactly-one-probe invariant, preventing a flapping endpoint from affecting probe admission for other consumer URLs.

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[Olasunkanmi975]

Hi maintainer, please review my work for Approval

[Olasunkanmi975]

@Jagadeeshftw hi maintainer please review my pr

[Jagadeeshftw]

good fix - the in-memory breaker was tracking half-open probe state globally rather than per url, so a probe for one consumer endpoint could block admission for a completely different url. now probe tracking is isolated per consumer url to match the redis implementation. the concurrency tests covering per-url isolation and the parity checks with redis semantics are exactly what this kind of shared-state bug needs.