fix: send and store real sub-second emission durations

[davidberenstein1957]

Extracted from #1203 (feat/add-fastapi-middleware), which bundled this with unrelated FastAPI middleware work. It stands alone and can be reviewed and merged independently; #1203 will be rebased to drop the duplicated hunks.

What

• duration becomes float instead of int in codecarbon/core/schemas.py and carbonserver/carbonserver/api/schemas.py (EmissionBase).
• ApiClient.add_emission no longer refuses emissions shorter than one second, and no longer truncates the duration with int(...).
• ExperimentReport, ProjectReport and OrganizationReport widen duration from int to float.

Why

The DB column and the ORM were always Column(Float) (carbonserver/carbonserver/api/infra/database/sql_models.py) — only the pydantic models claimed int. So nothing about storage changes here; the schemas are simply being made honest about what the database already holds. On the client side the < 1 guard silently dropped any measurement shorter than a second, which is every short-lived task.

The report widening is not cosmetic: those endpoints SUM() a Float column into a field declared int. Today every stored duration happens to be a whole number, so it validates by luck. The first sub-second duration in the database makes the sum non-integral and the response model raises a validation error — a latent 500 on the experiment/project/organization report endpoints. Widening the reports is therefore part of this fix, not a separate cleanup.

Tests

• carbonserver/tests/api/test_schema_compatibility.py::test_millisecond_duration_survives_client_to_server — a client payload with duration=0.0042 validates against the server schema unchanged.
• tests/test_api_call.py::TestApi::test_add_emission_sends_millisecond_duration_unchanged — replaces test_add_emission_skips_short_duration; asserts the POST body carries 0.0042.

Both fail on master (ValidationError, and emissions not sent because of a duration smaller than 1) and pass with the fix.

uv run pytest tests/ -q --ignore=tests/test_viz_data.py → 626 passed, 21 skipped. carbonserver unit tests → 108 passed. pre-commit run --all-files clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.43%. Comparing base (065d0e6) to head (54387a5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1374      +/-   ##
==========================================
+ Coverage   91.39%   91.43%   +0.03%     
==========================================
  Files          49       49              
  Lines        5056     5053       -3     
==========================================
- Hits         4621     4620       -1     
+ Misses        435      433       -2     

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