Fix schema filtering for parameter references

[ZevyaDev]

When include_tags or exclude_tags is configured, schemas referenced only by query, path, or header parameters are incorrectly treated as unused and omitted from generated output.

This causes generated clients to reference missing model files, for example:

Target of URI doesn't exist: '../models/unit.dart'
Undefined class 'Unit'

Changes:

• Track $ref schemas used by OpenAPI 3 parameters before type generation.
• Track $ref schemas used by OpenAPI 2 parameters as well.
• Preserve parameter-referenced enums and models during tag-based schema filtering.
• Add a regression test covering an enum referenced only by a filtered endpoint’s query parameter.

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[ZevyaDev]

Uh, I didn't ask you to review, stupid bot.