fix: use get_project_base() in Extension.validate()

[Thanhphan1147]

Extension.validate() is still using the raw base value from yaml_data:

base: str = self.yaml_data["base"]

However, with the new extension dispatch, _FrameworkFactory.__call__ uses get_project_base, which gives build-base precedence over base and more importantly use the value of build-base when base is bare. This breaks some workflow where the user sets build-base: ubuntu@26.04 and base: bare and rockcraft would report:

rockcraft.errors.ExtensionError: Extension 'expressjs-framework' does not support base: 'bare'

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[Copilot]

Pull request overview

This PR aligns Extension.validate() with the extension factory dispatch logic by using get_project_base() (which gives build-base precedence over base and normalizes legacy ubuntu:<series> syntax). This prevents extensions from incorrectly rejecting projects that use base: bare alongside a real Ubuntu build-base.

Changes:

• Update Extension.validate() to resolve the effective base via get_project_base() (falling back to the raw base field when it can’t be resolved).
• Adjust unit test expectations to match get_project_base() normalization of legacy ubuntu:<series> format.
• Add a regression test ensuring base: bare + build-base: ubuntu@26.04 dispatches to ExpressJS V2 without raising an extension base incompatibility error.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
rockcraft/extensions/extension.py	Use get_project_base() in Extension.validate() so validation matches dispatch rules (notably build-base precedence).
tests/unit/extensions/test_extensions.py	Update parametrization and expected error message to reflect base normalization (: → @).
tests/unit/extensions/test_expressjs.py	Add regression test for base: bare + build-base: ubuntu@26.04 dispatch/validation behavior.

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