fix: harden third-party tool result serialization

[wolfib]

Summary

Hardens McpPage.executeThirdPartyDeveloperTool against CDP returnByValue serialization failures and edge-case values returned by third-party developer tools.

Previously, if a tool returned values that CDP Runtime.evaluate (returnByValue: true) could not serialize (such as Symbol, BigInt, circular arrays, deeply nested objects, or Object.create(null)), pptrPage.evaluate either threw or resolved to undefined, resulting in TypeError: Cannot read properties of undefined (reading 'stashed') and leaving window.__dtmcp.stashedElements uncleaned.

Changes

• In-page JSON serialization: Stringify the processed tool result inside pptrPage.evaluate and JSON.parse it in Node so CDP only transfers a flat {result?: string, stashed: number} object by value.
• Improved processToolResult handling:
  • Track active ancestors using a Set with try ... finally cleanup across both arrays and plain objects, fixing circular array recursion while preserving shared (non-circular DAG) references.
  • Treat Object.create(null) (proto === null) as a plain object and fall back to <Object instance> for non-plain objects with missing or anonymous constructors.
  • Convert symbol and bigint values to string representations.
• Defensive cleanup & error handling:
  • Throw a descriptive error if pptrPage.evaluate resolves to undefined.
  • Ensure window.__dtmcp.stashedElements is only populated after serialization succeeds and is always cleared in a finally block.
  • Avoid appending undefined to response lines when a tool returns undefined.

[OrKoN]

Please check the following:

1. Remove the if (!result) guard, the | undefined return annotation on the evaluate callback in src/McpPage.ts, and the matching throws a descriptive error ... when evaluate resolves to undefined test in tests/McpPage.test.ts. Puppeteer only turns evaluate into undefined for "Object reference chain is too long" or "Object couldn't be returned by value" (see rewriteError in cdp/ExecutionContext.ts), and the new flat {result: string, stashed: number} return value can't trigger either one, so this branch can't run, its error message is misleading, and the test covers a case that can't happen (AGENTS.md: "Only test real scenarios").

2. In executeThirdPartyDeveloperTool (src/McpPage.ts), don't let the cleanup pptrPage.evaluate in finally replace the original error, and skip it when result.stashed === 0. If evaluateHandle fails because the page navigated or closed, the cleanup call fails too and its error hides the real cause; the page now only sets stashedElements when it's non-empty, so the extra CDP round trip on every call with no stashed elements isn't needed.

3. When the tool returns undefined, add an explicit response line (e.g. Tool returned no result.) in src/McpPage.ts instead of adding nothing. Right now the agent gets an empty tool response with no sign that the call succeeded, which is easy to read as a failure.