fix(sandbox-e2b): reliable exec timeout with server-side kill

[chcodex]

Fixes #2974.

• Per-call client disables the idle read timeout (readTimeout=0) so total-duration semantics is owned solely by callTimeout; previously a small base readTimeout preempted callTimeout and misreported a short idle stall as a full exec timeout (both surface as InterruptedIOException).
• Send Connect-Timeout-Ms so envd kills the remote process server-side; previously a client timeout only dropped our HTTP stream and leaked the process.
• External cancellation (Thread.interrupt()) is rethrown as-is with the interrupt bit restored instead of being wrapped as ExecTimeoutException.
• timeoutSeconds <= 0 now fails fast with IllegalArgumentException.
• Tests: E2bEnvdProcessClientTest 10/10 pass (4 new: header carried end-to-end, interruption rethrown with bit restored, fail-fast without request; plus existing codec/drain cases). Full reactor test BUILD SUCCESS.

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...e/extensions/sandbox/e2b/E2bEnvdProcessClient.java	84.61%	1 Missing and 1 partial ⚠️

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[CryoThrust]

One small robustness concern: res.body().string() is now unconditional in the non-success response path. OkHttp permits a response with a null body (for example a 204/transport edge or a test double), so this can replace the intended SandboxRuntimeException with an NPE. Keeping the previous null-safe fallback (res.body() != null ? res.body().string() : "") would preserve the error contract while still including the server body when present. A regression test with a non-success response and no body would pin this.

[chcodex]

@CryoThrust Thanks for flagging this. The null guard was removed deliberately:

• Response.body is a non-null contract in OkHttp (Kotlin val body: ResponseBody). For a real Call.execute() response — the only path that reaches this code — an empty body (204/304, truncated gateway reply) arrives as EmptyResponseBody, so .string() returns "" rather than throwing NPE.
• A null body can only come from a hand-built Response.Builder.build() without setting a body, which cannot occur on this production path. IDEA's nullability inference (from OkHttp's annotations) agrees and reports res.body() != null as always-true, i.e. the guard is dead code.
• The suggested regression test (non-success response with no body) would only exercise such a hand-built double, not a real server reply, so it would pin behavior that cannot happen in production.