fix: give CompiledSearchTerm a None related_terms default

[Kevin Turcios (KRRT7)]

Summary

• CompiledSearchTerm was a stdlib @dataclass subclassing the pydantic dataclass SearchTerm. A stdlib dataclass takes the base class's class-level value as the field default, and for related_terms that value is a pydantic FieldInfo object, not None.
  • CompiledSearchTerm(Term("x")).related_terms was therefore a truthy FieldInfo, and dedupe_related_terms crashed on it with AttributeError: 'FieldInfo' object has no attribute 'text'.
  • Production code only creates CompiledSearchTerm via cast, so it isn't hit there today; tests work around it by always passing related_terms=None.
• Declare CompiledSearchTerm with typeagent's pydantic dataclass decorator, like SearchTerm (same import as universal_message.py).
• A scan of all 103 dataclasses in the package found no other non-pydantic dataclass inheriting a FieldInfo default.

Relation to #348: #348 changes this default to Field(default=None, ...), which is still a FieldInfo, so the bug remains there. This PR doesn't touch any file #348 changes; it applies cleanly on top of #348 and the affected tests pass there too.

Validation

• New tests: a default-constructed CompiledSearchTerm has related_terms is None, and dedupe_related_terms accepts such terms. Both fail without the fix.
• make format check test

[Bernhard Merkle (bmerkle)]

LGTM — verified locally.

• Confirmed the root cause: a stdlib @dataclass subclass picks up SearchTerm's class-level FieldInfo as the related_terms default; both new tests fail against main's query.py and pass with the fix. Full make is green (813 passed).
• Checked the pydantic-dataclass semantics this switches to: term keeps its identity (important, since search mutates terms in place), the cast-based to_required_search_term path is unaffected, bad input now raises ValidationError, and post-construction assignment stays unvalidated, all the same as SearchTerm.
• FYI: a related_terms list passed to the constructor is now copied (elements keep identity), matching SearchTerm; nothing in src//tools/ constructs CompiledSearchTerm directly, so no impact.

(An equally small alternative would have been keeping the stdlib dataclass and redeclaring related_terms: list[Term] | None = None in the subclass, but matching SearchTerm's decorator is the more consistent choice.)