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)

operationHelpers.ts

• Add getMethodParamExpr() to build property accessor expressions from methodParameterSegments
• Refactor getOptional, getRequired, getPathParamExpr, buildHeaderParameter to use the new accessor logic
• Handle required vs optional method params correctly (no spurious options?. prefix on required body params)

Tests

• Unskipped "Should handle parameter grouping when using @@override" unit test
• Added 4 new bodyMetadataExtraction unit tests (required/optional x header/path in body models)
• All 23 targeted unit tests passing

Fixes #3540