Bound the BEP artifact-upload wait with a timeout (#2843)

### Description

Adds a `timeout` parameter to `execute_command()`, passed through to
`subprocess.run`. `upload_test_logs_from_bep()` uses it on the
`bazelci-agent artifact upload` call with a 2 hour limit; on timeout the
process is killed, the failure is logged, and the function returns
without re-raising, since the build's pass/fail result is already
decided by `bazel test` before this step runs -- a killed upload only
loses debug artifacts, not the build outcome. The same timeout is
applied to the `java.log` upload in the same function, which had the
same unbounded `execute_command()` call.

The 2 hour value is an estimate, not a measurement. The only
normal-runtime data available is from rules_scala itself (macOS
`test_rules_scala`: 40-55 min); other projects on this shared fleet may
run longer, worth confirming with maintainers before treating this as
final. A real fix inside `bazelci-agent` itself (detecting that the
writer process died) needs a new agent release; out of scope here.

### Motivation


[rules_scala#1906](https://github.com/bazel-contrib/rules_scala/issues/1906)
found jobs that stall inside `upload_test_logs_from_bep()` until
Buildkite's 8 hour step timeout kills them. This stays invisible because
the build still ends up green (a fast retry on some builds, `soft_fail`
on others), so the job burns most of 8 hours doing nothing and nobody
notices.

The exact hang mechanism inside `bazelci-agent` remains unverified --
the leading theory (bazel exits before writing `lastMessage`, so the
agent waits forever) failed to reproduce with the real binaries. That's
why this bounds the wait from the outside instead of targeting a
specific mechanism.

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Co-authored-by: bor-p-s <bpapevis@virtusity.com>