Support metadata parameters in body models and use methodParameterSegments for accessor paths

[JialinHuang803]

Problem

Two issues in the Modular emitter prevented correct handling of body models containing metadata properties (@header, @query, @path):

1. emitModels.ts: The isMetadata() filter unconditionally removed all metadata properties from model interfaces. When a body model contains @header or @query properties, these were stripped from the generated interface — making it impossible for users to pass those values.

2. operationHelpers.ts: Parameter accessor paths were built using bare param.name instead of walking TCGC's methodParameterSegments. This produced incorrect code when parameters are nested inside body models or grouped via @@override.

As a result, the @@override decorator's group parameters feature (issue #3540) is also resolved, since grouping @query/@header params into a model is a specific case of metadata properties in a body model.

Fix

emitModels.ts

• Conditionally keep metadata properties for input models (users need to pass these values)
• Filter metadata for output/exception models (deserialized separately via response headers)
• Add ARM resource model workaround: skip inherited @path name property (handled by ARM infrastructure)
• Add isArmResourceModel() helper to detect ARM resource base types by walking ancestor chain

operationHelpers.ts

• Add getMethodParamExpr() to build property accessor expressions from methodParameterSegments
• Add getParamAccessor() as a unified entry point for resolving parameter accessor paths (client-level, method-level with segments, or fallback)
• Refactor getOptional, getRequired, getPathParamExpr, buildHeaderParameter, getCollectionFormatForParam to use the new paramAccessor string instead of reconstructing paths from optionalParamName + param.name
• Handle required vs optional method params correctly (no spurious options?. prefix on required body params)
• Fix optional path parameter accessor to use options?.param instead of options["param"]

Tests

• Unskipped "Should handle parameter grouping when using @@override" unit test
• Added 5 new bodyMetadataExtraction unit test scenarios:
  1. Required @header + @query in @bodyRoot model
  2. Required @path in body model
  3. Optional @header + @query in @bodyRoot model
  4. Optional @path in body model
  5. Optional body model with @header + @query metadata
• Fixed existing tests for client-level header params, optional path params, and parameter normalization

Fixes #3540

[qiaozha]

wonder if you could add some test cases where the property has been moved to client via @clientLocation parameter.

[JialinHuang803]

This case is not supported yet and we couldn't get sufficient information from TCGC. There is no spector test case for it as well. I think we can put it as low priority since there is no real case from services.