Fix: Exclude required headerCollectionPrefix params from operation signature and respect generate options

[joheredi]

1. headerCollectionPrefix required parameter fix

Problem: When a required header parameter is decorated with @@clientOption("headerCollectionPrefix", ...), it was incorrectly included as a positional parameter in the operation method signature. However, the serialization code in the operation body already correctly skips this parameter — resulting in a required parameter that is never used.

Root Cause: In getOperationSignatureParameters() (operationHelpers.ts), the filter that builds the method signature excluded constants, client params, cookies, and auto-generated headers — but did not exclude headers with headerCollectionPrefix.

Fix: Added a single filter condition in getOperationSignatureParameters():

getClientOptions(p, "headerCollectionPrefix") === undefined

This works because SdkMethodParameter extends DecoratedType, so getClientOptions() can read the @clientOption decorator directly from the method parameter.

2. Respect generateTest and generateMetadata options

• Guard snippets.spec.ts generation with option.generateTest !== false so test files are not emitted when tests are disabled.
• Guard metadata.json emission with option.generateMetadata !== false so metadata is not emitted when explicitly opted out.

Testing

• Added new unit test scenario: headerCollectionPrefixRequiredSkipSignature.md — verifies a required metadata header with headerCollectionPrefix is excluded from both the send function and public operation signatures.
• All 581 existing unit tests pass.