Fix data race on Agent.pendingBindingRequests

[MiniNova]

What

Serialize all accesses to Agent.pendingBindingRequests with a dedicated
sync.Mutex.

Root cause

pendingBindingRequests (the LRU of outbound Binding-request transaction IDs)
is mutated from several places:

• sendBindingRequest — invalidatePendingBindingRequests + append
• handleInboundBindingResponse — invalidatePendingBindingRequests + removal
• updateConnectionState (on Failed) — reset to make([]bindingRequest, 0)
• revokeConsent — slices.DeleteFunc

These read-modify-write the slice header and its backing array. Under
go test -race, a suite that creates/tears down many short-lived agents
intermittently reports a data race on this field:

WARNING: DATA RACE
Write at ... by goroutine N:
  (*Agent).invalidatePendingBindingRequests() agent.go:...
  (*Agent).sendBindingRequest() agent.go:...
Current and previous write share the same call stack.

Fix

Add pendingBindingRequestsMu sync.Mutex and hold it across every mutation.

invalidatePendingBindingRequests is intentionally left unlocked: it is only
called from sendBindingRequest and handleInboundBindingResponse, both of
which already hold the mutex (locking inside it would deadlock).

How tested

• go build ./... and go vet ./... clean.
• Against a downstream SFU test suite under -race, the failing scenario
  (TestCallAliceBob) reproduced the race before the change and passed
  15/15 runs after it.

Notes

No API or behaviour change; only synchronization around an internal cache.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.70%. Comparing base (79c0411) to head (7c706d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1040      +/-   ##
==========================================
- Coverage   91.79%   91.70%   -0.10%     
==========================================
  Files          45       45              
  Lines        5450     5458       +8     
==========================================
+ Hits         5003     5005       +2     
- Misses        447      453       +6     

Flag	Coverage Δ
go	91.70% <100.00%> (-0.10%)	⬇️

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