Fix optional query/header parameters with constant values being hardcoded

[Copilot]

getParameterMap in the modular emitter unconditionally hardcoded constant-type parameters, ignoring optionality. This meant optional params like @query("name") foo?: "literal" or @header("name") bar?: "literal" always emitted the literal value instead of reading from options, making them impossible to omit.

Changes

• operationHelpers.ts: Gate the constant-value early return on !isOptional(param) so optional constants flow through the standard getOptional path, which reads from the options bag and preserves undefined for URL template omission and header conditional spreading.

- if (isConstant(param.type)) {
+ if (isConstant(param.type) && !isOptional(param)) {

Before (generated):

"optional%2Dquery%2Dname": "optionalQuery",  // always sent

After (generated):

"optional%2Dquery%2Dname": !options?.optionalQueryName
  ? options?.optionalQueryName
  : "optionalQuery",  // omitted when undefined

• New test scenarios covering optional constant parameters across all parameter kinds:
  1. queryParam/optionalConstantQueryParam.md — Optional constant query parameters with path and body params
  2. headerParam/optionalConstantHeaderParam.md — Required constant, optional constant, and optional string header parameters with body
  3. bodyParam/optionalConstantBodyParam.md — Optional constant body parameter

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Modular] Optional query parameters with constant values should not be hardcoded</issue_title>
<issue_description>### Problem
When generating operations with optional query parameters that have constant literal types, the generated code hardcodes the constant values instead of reading them from the options. This prevents users from omitting these optional parameters.

Reproduction

TypeSpec

op read(@path pathParam: string,
    @path("path-name") pathName: "path",
    @path("optional-path-name") optionalPathName?: "optionalPath",
    @query("query-name") queryName: "query",
    @query("optional-query-name") optionalQueryName?: "optionalQuery",
    @query("key-version") keyVersion?: string,
    @body parameters: string): OkResponse;

Generated Code (Incorrect)

export function _readSend(
  context: Client,
  pathParam: string,
  parameters: string,
  options: ReadOptionalParams = { requestOptions: {} },
): StreamableMethod {
  const path = expandUrlTemplate(
    "/{pathParam}/{path-name}{/optional-path-name}{?query%2Dname,optional%2Dquery%2Dname,key%2Dversion}",
    {
      pathParam: pathParam,
      "path-name": "path",
      "optional-path-name": "optionalPath",  // Seems correct because path should not be optional?
      "query%2Dname": "query",
      "optional%2Dquery%2Dname": "optionalQuery",   // ❌ HARDCODED
      "key%2Dversion": options?.keyVersion,  // ✅ CORRECT: Uses options
    },
    {
      allowReserved: options?.requestOptions?.skipUrlEncoding,
    },
  );
  // ...
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [Modular] Optional query parameters with constant values should not be hardcoded #3621

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[JialinHuang803]

@copilot Update the unit test to cover header and body parameters.

[Copilot]

@copilot Update the unit test to cover header and body parameters.

Added two more test scenarios in commit 73cacc7:

1. Optional constant header parameters — covers required constant ("requiredValue"), optional constant ("optionalValue"), and optional string header types, all alongside a @body parameter
2. Combined optional constant query + header with body — tests both @query and @header optional constants together with @body in a single operation

All 538 modular unit tests pass.

[JialinHuang803]

@copilot the test is in the queryParam folder, while it contains header param test scenarios. Can you remove the test "Required constant body with optional constant query and header" and move to header test scenario to a correct place? Also add a test for optional constant body parameter.

[Copilot]

@copilot the test is in the queryParam folder, while it contains header param test scenarios. Can you remove the test "Required constant body with optional constant query and header" a...

Done in ba603b4:

1. Removed the "Required constant body with optional constant query and header" combined test from queryParam/
2. Moved the header test to headerParam/optionalConstantHeaderParam.md
3. Added optional constant body parameter test at bodyParam/optionalConstantBodyParam.md

All 538 modular + 309 RLC unit tests pass.

[MaryGao]

Approved and left small comments.