[Misc] Remove stale level reference from CompilationConfig validator

[UgaTheDev]

CompilationConfig.level was renamed to mode, but two references to the old name were left behind:

• _skip_none_validation still lists "level" in its @field_validator field list. The field no longer exists on the class, so the entry is dead. Pydantic ignores a validator naming an unknown field on a dataclass, so this is not a runtime error today, just a stale reference that is misleading to read.
• The ValueError raised when mode is unset still says "where the level is set".

This drops the dead entry and corrects the message to say mode. No behaviour change.

[claude]

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[UgaTheDev]

This one is stuck behind pre-run-check, which gates CI until an author has 4 merged PRs. My first merged PR landed yesterday (#51627), so the count is 1 and this cannot run its own tests yet.

Could a maintainer kick it off with /ci run? The change is 1 insertion and 2 deletions: CompilationConfig.level was renamed to mode, and the _skip_none_validation field validator still lists the old name along with a stale mention in an error message. Pydantic ignores a validator naming a field that does not exist, so there is no behaviour change, it is dead-reference cleanup.