refactor(runtime): keep process identity API minimal

[ScriptedAlchemy]

Summary

Follow-up to merged #396.

• remove the unused public random_hex_token(bytes) helper and its test
• remove comments added only to justify why existing token generators did not call that unused helper
• retain the actual shared process_run_id() API used by MCP analytics

Why

The helper had no production caller and its RNG-failure fallback always returned a 16-character timestamp regardless of the requested byte count, contradicting its documented 2 * bytes contract. Keeping it would publish speculative surface area with an untested failure-path bug.

Tests

• cargo test --lib runtime_identity: pass
• cargo test --lib mcp::: 183 immediate tests passed; unrelated timer/global-store tests then starved under the concurrent local build load and the run was interrupted
• upstream refactor: shared process run id and random-token minting #396 CI was green before merge; this follow-up only deletes unused code/comments

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