ci: behavioral tests for the bazel-retry wrapper

[b7r6]

What and why

Follow-up to #2897: give tools/bazel-retry.sh a behavioral test harness so
this copy can serve as the canonical one, with the retry/no-retry contract
pinned by tests rather than by reading the regex. (A second copy of this
wrapper lives in an internal repo; unifying the two is tracked internally,
and pinning this copy's behavior gives that comparison a fixed point.)

Also drops a misleading exec in the wrapper: inside the pipeline it only
ever replaced the subshell, so behavior is unchanged, but it reads like a bug
to anyone auditing the retry flow.

How was this verified?

python3 tools/test-bazel-retry.py (stdlib only, no bazel needed). It runs
the real script with a fake bazel shimmed onto PATH that plays a scripted
per-attempt plan and logs every invocation, so each test asserts both the
exit code and exactly how many attempts the wrapper made. Covered: first-try
success; transient HTTP 503 retry; the #2897 remote-endpoint patterns
(capabilities-query connection-refused, bare UNAVAILABLE:); a compile error
exiting 1 after exactly one attempt (the invariant that keeps red builds from
costing 3x CI); budget exhaustion after three transient failures; and argv
pass-through with embedded spaces. The attempt-count assertions fail against
a wrapper that retries unconditionally, and the suite takes ~30s because the
real 5s/10s backoff sleeps run.

Not covered here: reconciling the internal copy of the script and its test
harness. This PR just pins this copy's behavior so that comparison has a
fixed point.

Risk

Low. The wrapper change is a no-op exec removal on a CI-only script; the
test file is new and runs nowhere automatically yet (happy to wire it into a
workflow here or leave that for the follow-up, maintainer's call).

AI assistance

Claude (Fable 5) drafted the harness and the exec removal from my direction;
I reviewed every line before opening this.

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