feat(emissions): add UK National Grid ESO live carbon intensity source

[sufirehman]

Description

Adds codecarbon/core/national_grid_eso_api.py, which calls the public National Grid ESO Carbon Intensity API (https://api.carbonintensity.org.uk/intensity) to retrieve half-hourly live carbon intensity for Great Britain. The actual reading is preferred; forecast is used as a fallback when actual has not yet been published. A genuine zero-intensity actual value is handled correctly via an explicit is not None check rather than a truthiness test.

Integrates the new source into the get_private_infra_emissions fallback chain in emissions.py immediately after ElectricityMaps: when geo.country_iso_code == "GBR" and no ElectricityMaps token is configured, the live API is tried first and the existing static country-average remains the final fallback on failure. No behaviour changes for any other country.

Related Issue

Closes #947

Motivation and Context

UK grid carbon intensity varies significantly throughout the day depending on wind and solar generation, but the existing static yearly average can't reflect this. This was raised directly in #947 by a maintainer. Live UK data improves estimate accuracy for any user running compute in Great Britain, without changing behaviour for any other country.

How Has This Been Tested?

Added unit tests in tests/test_national_grid_eso_api.py covering the happy path, fallback to forecast when actual is None or absent, HTTP error handling, and malformed response handling, all using the responses library to mock HTTP calls in the same style as the existing test_electricitymaps_api.py. Also included a skipped integration test, and manually verified it against the real live API locally (confirmed a real positive emissions value returned in under 2 seconds). Ran the full existing core test suite to confirm no existing tests were broken.

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[sufirehman]

Hi maintainers, wanted to follow up on PR #1257. This adds live UK National Grid ESO carbon intensity data as a real-time source, replacing the static 2023 annual average for UK measurements, directly addressing issue #947.
I've been actively using CodeCarbon in my own research (IEEE EEEIC 2026 paper on carbon-aware ML scheduling) and this change matters practically, the UK grid carbon intensity varies significantly hour to hour, and half-hourly live data produces much more accurate emissions accounting than a static annual figure.
Happy to make any changes requested to get this across the line. Is there anything blocking review?

[sufirehman]

Hi, just noticed the branch sync from @inimaz, thanks for keeping this current. Happy to address any feedback or make adjustments if anything is blocking the review.

[davidberenstein1957]

Thanks for the patience on this one. The source itself is useful (public API, no key, national UK intensity). The blocker is that it is not opt-in: with this PR every run with country_iso_code == "GBR" makes a blocking network call with a 30 s timeout before falling back. Users on air-gapped or slow machines would see runs stall.

Asks: gate it behind a config/env option (the Electricity Maps integration is the pattern), use a short timeout (a few seconds), cache the value across measurements within a run, and add the API's attribution terms to the docs. It is also national-level, not regional, which the docs should say.