[pyrefly] Deduplicate legacy type parameter quantification

[lyydsheep]

Fixes #4299

Share the existing quantified construction for TypeVar, TypeVarTuple, and ParamSpec across the two legacy type-alias conversion paths while keeping diagnostics, source anchors, defaults, and substitutions unchanged.

Validation:

• cargo test -p pyrefly type_alias -- --nocapture (109 passed)
• cargo test -p pyrefly generic_legacy -- --nocapture (66 passed)
• cargo test -p pyrefly type_var_tuple -- --nocapture (39 passed)
• python3 test.py --no-test --no-tensor-shapes --no-conformance --no-jsonschema

The local .scratch planning files are intentionally not part of this PR.

[meta-codesync]

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