[FFL-2929] Add assignment request timeout

[leoromanovsky]

Motivation

Flag assignment requests can wait on a slow or incomplete response without an SDK-level bound.

Changes and Decisions

• Add an opt-in timeout for each assignment request. Zero preserves current behavior.
• Include the complete response-body download in the timeout.
• Preserve a shorter timeout from the HTTP transport.
• Support an assignment-only custom Call.Factory. The SDK timeout composes with its calls.

Validation

• Android 16 / API 36 emulator timeout matrix: 15/15 passed

[datadog-datadog-prod-us1-2]

✅ All CI checks and tests passed. Datadog automation helped this PR pass.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog retried 1 test - 1 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 96.00%
• Overall Coverage: 71.86% (+0.13%)

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[leoromanovsky]

Fresh live timeout validation

Validated 949113041 on a Google Android 16 / API 36 emulator with a deterministic local HTTPS fault server. Each timeout-only case ran three times through public Flags APIs. Telemetry uploads were disabled.

Validation	Expected behavior	Fresh observed result
Timeout disabled	A 1.2-second response succeeds without an SDK timeout.	3/3 passed; one request; one completion; ready in 1,219–1,254 ms.
Full-body timeout	A stalled response body stops near 500 ms.	3/3 passed; one request; one completion; error in 503–507 ms.
Initial request only	HTTP 500 ends after the initial assignment request.	3/3 passed; one request; one completion; error in 50–53 ms.
Custom Call.Factory	Apply the public 500 ms timeout to a custom factory.	3/3 passed; one factory call; each call received 500 ms; error in 504–505 ms.
Shorter transport timeout	Preserve a custom 250 ms timeout under the 500 ms SDK timeout.	3/3 passed; one factory call; timeout remained 250 ms; error in 252–255 ms.

Result: 15/15 fresh timeout cases passed.