FEAT: Add In-Flight Concurrency Limiter Middleware (Bulkhead)

[MrEthical07]

Background:
Rate limiting controls request rate, but not concurrent in-flight pressure on expensive handlers.

Problem:
Under bursty traffic, high concurrency can still saturate downstream dependencies despite rate limits.

Scope:

• Add optional global middleware to cap concurrent in-flight requests.
• Integrate in internal/core/httpx/globalmiddleware.go.
• Add tests and benchmark impact in internal/core/httpx/globalmiddleware_test.go and internal/core/httpx/benchmark_test.go.

Tasks:

• Implement configurable concurrency cap middleware.
• Define overflow behavior and response status.
• Add metrics hooks for accepted/rejected counts.
• Add tests for cap enforcement and recovery.

Acceptance Criteria:

• Requests above configured cap are rejected predictably.
• Middleware is disabled by default unless configured.
• Load tests show improved stability under burst concurrency.