Add treat-unknown-as-record feature flag to map TypeSpec unknown to Record<string, unknown> in Modular SDK

[Copilot]

When migrating from HLC to TypeSpec-generated Modular SDKs, TypeSpec unknown emits as any, but HLC mapped the equivalent swagger type to Record<string, unknown>. This breaks public API compatibility for migrating services.

Changes

• New emitter option treat-unknown-as-record (boolean, Modular only, default false): when enabled, TypeSpec unknown maps to Record<string, unknown> instead of any
• Type resolution (get-type-expression.ts): unknown case now checks context.rlcOptions?.treatUnknownAsRecord to select the output type
• No-wrap behavior (operationHelpers.ts): checkWrapNonModelReturn now returns noWrap for unknown type when treatUnknownAsRecord is enabled, treating it as a Dictionary/Record type (consistent with HLC's PropertyKind.Dictionary → no body wrapper behavior)
• Option wiring: threaded through EmitterOptions → RLCOptionsSchema → RLCOptions → transfromRLCOptions.ts following the existing pattern used by similar compat flags
• Test utilities: emitModularModelsFromTypeSpec and emitModularOperationsFromTypeSpec wired to forward the new option
• Scenario tests: added treatUnknownAsRecord.md covering enabled/disabled flag behavior and the noWrap interaction with wrap-non-model-return

Example

With treat-unknown-as-record: true in tspconfig.yaml:

// Before (default)
export interface Foo {
  bar: any;
  baz: any[];
}

// After (treat-unknown-as-record: true)
export interface Foo {
  bar: Record<string, unknown>;
  baz: Record<string, unknown>[];
}

When combined with wrap-non-model-return: true, the unknown return type is no longer wrapped in a { body: any } alias — it is returned directly as Record<string, unknown>:

// Before (wrap-non-model-return: true, treat-unknown-as-record not set)
export async function getAny(...): Promise<GetAnyResponse> { ... }
// GetAnyResponse = { body: any }

// After (both flags enabled)
export async function getAny(...): Promise<Record<string, unknown>> { ... }

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[MaryGao]

@copilot when the treatUnknownAsRecord is enabled, we should generate noWrap for unknown type. also add an UT for unknow as return type for treatUnknownAsRecord on/off.

[Copilot]

Done in commit 6d87a9f. When treatUnknownAsRecord is enabled, checkWrapNonModelReturn now returns noWrap for unknown type (treating it as a Dictionary/Record type, consistent with HLC's PropertyKind.Dictionary → no body wrapper behavior). The return type is Record<string, unknown> directly instead of being wrapped in { body: any }.