feat(http): add no-content-type-optionality-mismatch linting rule

[Copilot]

When a Content-Type header is required but the @body/@bodyRoot is optional (or vice versa), the mismatch can cause runtime issues—there's no content to describe when the body is omitted.

Adds linting rule @typespec/http/no-content-type-optionality-mismatch (warning) that checks operation parameters and reports when the optionality of a Content-Type header property diverges from its associated request body.

Example

// ❌ content-type required, body optional
op foo(
  @header("content-type") contentType: "application/json",
  @body body?: Bar,
): void;

// ✅ optionality matches
op foo(
  @header("content-type") contentType?: "application/json",
  @body body?: Bar,
): void;

Changes

• packages/http/src/rules/no-content-type-optionality-mismatch.ts — Rule implementation; resolves HTTP properties from operation parameters, finds content-type and body/bodyRoot properties, compares optionality
• packages/http/src/rules/no-content-type-optionality-mismatch.md — Extended rule documentation with incorrect/correct examples
• packages/http/src/linter.ts — Register the new rule
• packages/http/test/rules/no-content-type-optionality-mismatch.test.ts — 14 tests (5 diagnostic-emitting, 9 valid cases)
• packages/http/README.md, website/.../linter.md — Regenerated docs

• Fixes validate content-type set as constant/required while @body or @bodyRoot value is optional? #11413

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