fix(codex): preserve distinct requests when cumulative usage is missing

[roy-tong]

When total_token_usage is absent, the old cumulative guard and all-zero deduplication key collapse subsequent requests. Keying only on last usage also loses distinct requests with equal token amounts.

This revision:

• applies the cumulative guard only to a finite, nonnegative reported total_tokens;
• treats absent, null and partial totals as missing identity rather than zero;
• uses source path and physical record offset for fallback identity, preserving distinct equal-usage rows and stable same-file rereads;
• deliberately makes no cross-file replay-deduplication claim when cumulative identity is unavailable;
• invalidates both Codex result and provider/session caches so existing aggregates are reparsed.

Validation: 37 tests pass across the relevant Codex test files, including the new three-case regression and cache invalidation coverage; TypeScript --noEmit passes. The three missing-cumulative regressions fail on the previous PR head and pass with this revision. Fixtures are synthetic.