[Rust] Log selected backend log format at startup

[therealsaitama0]

Summary

Adds a startup log field for the selected backend log format (TOT_LOG_FORMAT) so operators can confirm text/json mode from startup logs.

Changes

• backend/src/logging.rs: New module with
  esolve_log_format() helper that reads TOT_LOG_FORMAT, defaults to ext, accepts json, and returns an error for invalid values.
• backend/src/lib.rs: Registers the new logging module.
• backend/src/main.rs: Calls
  esolve_log_format() before initializing tracing, selects JSON or plain text formatter based on the resolved format, and includes log_format in the startup info event.
• docs/OPERATIONS.md: Documents the TOT_LOG_FORMAT environment variable and the log_format startup confirmation field.

Testing

• cargo test --lib passes all logging unit tests:
  • defaults_to_text_when_unset
  • �ccepts_text
  • �ccepts_json

ejects_invalid_value

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

---

• I would like to request that my diagnostic build log is removed before merging

Payment

ETHEREUM ADDRESS: 0x5e1040927a1E28D740f92De27a3d493b81682D88