test(a2a): apply invocation context deltas in fake

[Sayt-0]

Summary

Fix the A2A invocation-context fake so WithICDelta applies the supplied delta instead of silently discarding it.

Issue expectations

Expectation	Implementation
Apply the delta consistently with WithContext, or fail loudly	WithICDelta returns a shallow copy and applies Context, UserContent, Agent, Branch, and IsolationScope
Preserve unaffected context state	Partial deltas retain all unspecified fields, and the source context is not mutated
Add regression coverage	TestFakeInvocationContext_WithICDelta covers complete, nil, partial, and explicit nil-content deltas
Verify the regression	Restoring the previous return c implementation makes the new test fail

Validation

• task build
• task test
• task lint
• go test -race ./pkg/a2a -count=1
• go vet ./pkg/a2a

Closes #4048