feat: Add upstream proxy timeouts, circuit breaking, and safe retries (Closes #1149)

[devyeyostellar]

Fixes #1149

• Safely wraps fetch with a retry mechanism for idempotent operations (GET, HEAD, OPTIONS) up to 3 times.
• Ensures the timeout deadline remains strict and the circuit breaker wraps the retry operation cleanly to avoid cascading failures.

[greatest0fallt1me]

Before merge, please add the missing acceptance coverage for #1149: tests for slow headers/resets, bounded retry limits, and circuit-breaker recovery through a successful probe. The current change only updates proxy execution logic and does not include those regression tests.

[devyeyostellar]

Thanks for the review @greatest0fallt1me! I have just pushed an additional commit that adds the missing acceptance coverage for slow headers/resets, bounded retry limits (verifying safe vs unsafe methods), and circuit-breaker recovery through a successful probe. Let me know if there is anything else needed!