Fix handling of absolute local schema paths from yaml.schemas and yaml/get/jsonSchema StackOverflow

[shin19991207]

What does this PR do?

1. Fixed handling of absolute local schema paths from yaml.schemas

   When multiple schemas with absolute local paths matched the same YAML file, the paths were kept as plain filesystem paths instead of being normalized to file:// URIs before schema registration.

   This causes problem when a YAML file matches multiple schemas:

   • yaml-language-server combines those schemas internally as schemaservice://combinedSchema/...
   • because the schema paths were not normalized first, the combined-schema resolution resolved each absolute local schema path /Users/.../schema.json against the internal schemaservice://combinedSchema/... URI, producing invalid references like schemaservice:///.../schema.json
   • schema loading failed with Unable to load schema from 'schemaservice:///...': No content.

   This PR fixes normalizes absolute local paths from yaml.schemas to file:// URIs before schema registration. This ensures schema registration uses the correct local file URI, so schema validation and schema selection (schema picker) works properly.

2. Fixed yaml/get/jsonSchema StackOverflow

   The problem was that the schema picker (status bar) in vscode-yaml calls yaml/get/jsonSchema only wanting to get schema metadata such as schema URI, name, description, and versions. However, the yaml/get/jsonSchema handler in YLS was using the full schema resolution path through the getSchemaForResource(). Since this path is intended for validation, it can resolve combined schemas and walk complex $ref graphs. When a YAML file matched multiple "large" or "complex" schemas, the metadata request could end up doing deep schema resolution just to render the UI, which could cause a StackOverflow:

   [Trace - 3:22:55 PM] Received response 'yaml/get/jsonSchema - (153)' in 22ms.
   Request failed: Request yaml/get/jsonSchema failed with message: Maximum call stack size exceeded (-32603).
   

   Since yaml/get/jsonSchema is used for schema status UI and only needs display metadata for the schemas associated with the current YAML doc, this PR adds a lightweight lookup path for that request to return metadata only without fully resolving the schemas.

What issues does this PR fix or reference?

Fixes redhat-developer/vscode-yaml#1249
Related to redhat-developer/vscode-yaml#1253

Is it tested? How?

• New automated tests
• Manual testing

To verify absolute local paths from yaml.schemas is registered correctly:

1. Map two or more JSON schemas with absolute local paths to the same YAML file:

"yaml.schemas": {
  "/Users/.../schema1.json": "/Users/.../asdf.yaml",
  "/Users/.../schema2.json": "/Users/.../asdf.yaml"
}

2. Go to /Users/.../asdf.yaml. Verify validation works and no error appears like Problems loading reference 'schemaservice:///...': Unable to load schema from 'schemaservice:///...': No content..
3. Open the schema picker from Multiple JSON Schemas, verify the schemas are shown as file:///... URIs, not schemaservice:///... URIs.

To verify yaml/get/jsonSchema does not cause StackOverflow:

1. Map multiple "large" JSON schemas to the same YAML file, e.g.:

"yaml.schemas": {
      "https://raw.githubusercontent.com/Azure/azure-dev/main/schemas/alpha/azure.yaml.json": "/Users/.../asdf.yaml",
      "https://www.schemastore.org/sourcehut-build-0.65.0.json": "/Users/.../asdf.yaml"
}

2. Open the schema picker, verify that there's no error appears like [Trace - 3:22:55 PM] Received response 'yaml/get/jsonSchema - (153)' in 22ms. Request failed: Request yaml/get/jsonSchema failed with message: Maximum call stack size exceeded (-32603).

[datho7561]

Looks good! Thanks, Morgan!