feat(client): update request timeout policy at runtime

[SunSi12138]

Closes #578

Scope

Adds runtime request-timeout policy replacement for future logical calls without rebuilding or reconnecting the client.

• ISharpLinkClient.UpdateRequestTimeout(TimeSpan) atomically publishes an enabled custom fallback.
• ISharpLinkClient.DisableRequestTimeout() atomically publishes a disabled fallback.
• GetRequestTimeoutPolicySnapshot() exposes the current immutable generation, source and effective timeout.
• candidates are validated before publication and updates are rejected once stop/drain/fault lifecycle begins.
• each logical call captures exactly one timeout-policy generation while resolving call control; the resulting deadline remains frozen across interceptor suspension, retries and streaming lifetime.
• explicit method-level timeout keeps precedence; parameterless method timeout still falls back to the captured client policy; inherited deadline behavior is unchanged.
• publication uses the existing client state gate only on mutation; the RPC hot path performs one volatile generation read and adds no new lock.

Regression

SharpLinkClientRuntimeRequestTimeoutTests covers:

• an in-flight call retaining its original 10s deadline after the client publishes a 1s generation, while a later call observes 1s;
• enable / invalid-candidate / disable publication and idempotent disable;
• explicit method timeout precedence and parameterless method fallback;
• concurrent enable/disable snapshots never exposing torn policy state;
• update rejection after stop begins.

Branch head: 27d13b6216b35ac2b678d41a4acb80902ee45269.
Target validated by PR merge ref: dev@f643aee53e411fa2a6825cf484787069468f9c64.

Validation

• PR Fast run 34131406970: PASS — Release build, full Unit/Generator/Load tests, formatting/maintainability and deterministic allocation gate.
• Ready-state PR Fast run 34132885891 attempt 2: PASS. The first attempt hit two unrelated existing timing flakes outside the changed Client timeout surface; rerun on the identical merge ref passed the full suite.
• CodeQL run 34131406982: PASS.
• PR Extended run 34132910880: PASS — Debug/Release builds, full Integration Tests, NativeAOT transport/topology smoke, package verification/smoke, demo/load smoke, and desktop cross-platform codec compatibility matrix.