fix: always treat path parameters as required per OpenAPI 3.x spec

[Himenon]

Summary

• Per OpenAPI 3.x spec §3.3.2, path parameters (in: path) are always required — the required field is REQUIRED and its value MUST be true.
• Added normalizePathParameters in generateValidRootSchema.ts to unconditionally set required: true on all path parameters, covering three locations: components.parameters, PathItem-level parameters, and operation-level parameters.
• Without this fix, path parameters that omit required: true in the spec would appear as required: undefined in pickedParameters and operationParams.parameters, violating the OpenAPI contract.

Test plan

• Added unit tests for generateValidRootSchema covering: operation-level path params, PathItem-level path params, components.parameters path params, query params (unchanged), and already-explicit required: true (unchanged).
• Added OpenAPI fixture test/path-parameter/index.yml with path parameters that omit required: true.
• Added snapshot tests (class + functional) verifying the generated Parameter interface has non-optional properties for path parameters (id: string, not id?: string), and pickedParameters carries required: true.
• All existing tests pass (pnpm test:vitest 40/40, pnpm test:snapshot 53/53).

Closes #148

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