fix: preserve exception tracebacks in logs

[Timandes]

Summary

• preserve exception type and traceback output in StandardFormatter when logging with exc_info
• add logging.exception_traceback_enabled YAML configuration and ROCK_LOGGING_EXCEPTION_TRACEBACK_ENABLE environment-variable override
• apply YAML logging configuration to the shared admin/proxy lifespan while keeping rocklet on environment/default behavior
• retain the current single-line log format when traceback output is disabled

Root cause

StandardFormatter.format() built the final message directly from record.getMessage() and did not append record.exc_info. Exceptions with an empty message, such as httpx.PoolTimeout(""), therefore produced only Error in http_proxy: with neither an exception type nor a traceback.

Impact

Exception tracebacks are enabled by default. Operators can restore the previous behavior with either:

logging:
  exception_traceback_enabled: false

or:

ROCK_LOGGING_EXCEPTION_TRACEBACK_ENABLE=false

An explicitly set environment variable takes precedence over YAML configuration.

Validation

• uv run pytest tests/unit/test_logger.py tests/unit/test_config.py tests/unit/admin/test_logging_config.py -q — 83 passed
• uv run ruff check rock/logger.py rock/env_vars.py rock/config.py rock/admin/main.py tests/unit/test_logger.py tests/unit/test_config.py tests/unit/admin/test_logging_config.py
• uv run ruff format --check rock/logger.py rock/env_vars.py rock/config.py rock/admin/main.py tests/unit/test_logger.py tests/unit/test_config.py tests/unit/admin/test_logging_config.py
• all three sample YAML files parsed and validated
• project fast profile: 2088 passed; 13 failed and 33 errored in Docker/Testcontainers and shell temporary-output tests because of the local test environment

Fixes #1260