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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