ci(samples): retry transient LLM 503/429 errors and propagate runner exit codes - #4442

Wrap sample test invocations in run_with_retry (up to 3 attempts with exponential backoff and DB table reset) when transient upstream LLM errors (503 UNAVAILABLE / 429 RESOURCE_EXHAUSTED) occur, and add matching retry helpers in the Go sample test harnesses. Also replace the piped find | while loop with process substitution and capture test exit codes before cleanup so subshell failures are never masked by rm -rf.

Description

Hardens sample integration tests against transient upstream Gemini API capacity errors (503 UNAVAILABLE / 429 RESOURCE_EXHAUSTED) and fixes exit-code masking in .ci/sample_tests/run_tests.sh:

• Adds run_with_retry and is_transient_llm_error in .ci/sample_tests/run_tests.sh to retry sample test runs up to 3 times with exponential backoff and a fresh setup_db_table reset when transient 503/429 LLM errors occur.
• Adds matching retry logic in docs/en/documentation/getting-started/quickstart/go/quickstart_test.go and docs/en/documentation/configuration/pre-post-processing/go/agent_test.go for standalone go test runs (disabled via SAMPLE_TEST_MAX_ATTEMPTS=1 when already wrapped by run_tests.sh).
• Captures test runner exit codes before rm -rf cleanup in run_python_test, run_js_test, and run_go_test, and replaces find ... | while read with while read ... done < <(find ... | sort) so failures inside the loop are not lost in a pipeline subshell.

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🛠️ Fixes #<issue_number_goes_here>