feat(openapi): serialize simple object path parameters - #433

Generated MCP tools can now serialize flat object path parameters with OpenAPI's simple style, including omitted style/explode defaults. { R: 100, G: 200 } expands to R,100,G,200, or R=100,G=200 with explode: true, instead of [object Object].

Keys and values are escaped separately before adding separators. Scalar/array encoding, unsupported nested/style fallback, and the existing substituted-dot-segment guard are preserved. The documentation and parameter comments describe the new support and remaining limits. This extends the parameter serialization work in #414 and addresses #418.

Validation on Windows with Node 22.20.0 and pnpm 10.32.1:

• All 261 offline OpenAPI tests pass, including generated MCP calls and 22 new cases. Before the fix, 12 new regression cases failed.
• Prettier, ESLint, TypeScript checking, and the package build pass.
• All 15 network OpenAPI tests pass, including both live Petstore checks. The upstream benchmark helper warned about an unstable PostHog spec hash and allowed the freshly fetched content; no fixture pins were changed.
• The full suite reports 1067 passed, 2 skipped, and two DiskStore concurrent-rename failures. Both failures also occur on the unmodified base (1045 passed, 2 skipped); they are not reported as passing.
• The combined pnpm lint command reaches and passes its first three checks, then fails in the existing Deno 1.41.3 JSR dry-run on dependency declaration files. The JSR entry points and their production inputs are unchanged. Linux/Node 24 CI has not been run locally.

AI assistance was used to prepare this change; the implementation and validation evidence received an independent agent review.