refactor: improve HttpApi authorizer validation

[vicheey]

Issue #, if available

Close #3817

Description of changes

Refactor HttpApi authorizer property validation in apigatewayv2.py from repetitive if/raise blocks to a data-driven approach using two declarative dicts (ALLOWED_PROPERTIES and PROPERTY_DISPLAY). This replaces
~40 lines of repetitive validation with a 4-line loop, making it easier to maintain and extend when new authorizer types or properties are added.

No behavior change — same properties are blocked for the same authorizer types. The only user-facing difference is improved error messages:

• Before: "IdentitySource must be defined only for OAuth2 Authorizer."
• After: "IdentitySource is only supported for OAuth2 Authorizer. For Lambda Authorizer, use the 'Identity' property instead."
  The IdentitySource property is intentionally only supported for OAuth2Authorizer per the SAM schema
  (https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-property-httpapi-oauth2authorizer.html). For LambdaAuthorizer, the Identity
  (https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-property-httpapi-lambdaauthorizationidentity.html) property (with Headers, QueryStrings, StageVariables, Context) is the
  correct way to set identitySource. The updated error message now guides users to this.

Description of how you validated changes

• All 17 existing unit tests in tests/model/test_api_v2.py pass (error message strings updated to match)
• make pr passes: black, ruff, mypy --strict, cfn-lint, schema generation all clean
• Manually deployed a REQUEST authorizer using the Identity property workaround and verified the authorizer was created with correct identitySource: ["$request.header.Authorization"] via aws apigatewayv2
  get-authorizers

Checklist

• Review the generative AI contribution guidelines (https://github.com/aws/serverless-application-model/blob/develop/CONTRIBUTING.md#ai-usage)
• Adheres to the development guidelines (https://github.com/aws/serverless-application-model/blob/develop/DEVELOPMENT_GUIDE.md#development-guidelines)
• Add/update transform tests (https://github.com/aws/serverless-application-model/blob/develop/DEVELOPMENT_GUIDE.md#unit-testing-with-multiple-python-versions)
• Using correct values
• Using wrong values
• Add/update integration tests (https://github.com/aws/serverless-application-model/blob/develop/INTEGRATION_TESTS.md)

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates (https://github.com/aws/aws-sam-cli-app-templates).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[licjun]

For this #3817 (comment), it seems customer is using the schema defined for cloudformation apigatewayv2, not SAM's schema for http api, since sam http api doesn't support AuthorizerResultTtlInSeconds, rather this cache config is embedded into Identity's ReauthorizeEvery

[licjun]

There is still some tests failed. Otherwise LGTM.