Resolve #1662: Add runtime edge regression coverage

[ayden94]

Summary

Closes #1662.

Adds executable regression coverage for documented @fluojs/runtime shutdown and multipart size fallback contracts without changing runtime behavior.

Changes

• Adds Node adapter integration coverage proving multipart requests fall back from multipart.maxTotalSize to maxBodySize.
• Adds Node request factory coverage for the same multipart fallback contract.
• Adds Web request dispatch coverage for the multipart fallback path.
• Adds signal-driven shutdown failure coverage for app.close(signal) rejection logging and process.exitCode handling.

Testing

• LSP diagnostics: clean for packages/runtime/src/application.test.ts, packages/runtime/src/web.test.ts, and packages/runtime/src/node/node-request.test.ts.
• pnpm --filter @fluojs/runtime exec vitest run -c vitest.config.ts src/application.test.ts src/web.test.ts src/node/node-request.test.ts
• pnpm --filter @fluojs/runtime typecheck
• Built runtime dependency chain before typecheck: pnpm --filter @fluojs/core --filter @fluojs/di --filter @fluojs/validation --filter @fluojs/http --filter @fluojs/serialization --filter @fluojs/runtime build

Public export documentation

• No public exports changed.
• Changed exported functions document matching @param / @returns tags where applicable.
• Source @example blocks and README scenario examples still play complementary roles.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

No release changeset is included because this is test-only regression coverage for existing documented runtime behavior.

Platform consistency governance (SSOT)

• SSOT English/Korean mirror structure remains synchronized for changed governance docs.
• If platform contract docs changed, companion updates include discoverability/docs index, tooling or CI enforcement, and regression-test evidence.
• Any package README alignment/conformance claims are backed by createPlatformConformanceHarness(...) tests.

No governance docs or package README contract text changed.