feat(mcp): expose x402 receipt metadata and explorer URL from paid MCP tools

[AGWAM001]

Closes #172

Summary

Extends paid MCP tool output in mcp-server/index.ts to include the versioned x402 receipt object and an explorer URL, alongside the existing amount/network reporting. Missing or unverified receipts are now represented explicitly rather than being folded into a generic "paid success" response.

Problem

MCP search output currently reports payment amount and network, but omits the actual verifiable receipt and a transaction link — so a caller has no way to independently verify that a reported payment actually settled, short of trusting the amount/network fields at face value. There's also no distinction in output between "payment verified with receipt" and "receipt missing/unverified" — both would currently look like a normal paid success.

Changes
Receipt object in output: paid tool responses now include the versioned x402 receipt object as returned/verified during settlement, not just the derived amount/network summary.
Explorer URL: added a transaction explorer link derived from the settlement result, so a caller can independently inspect the on-chain transaction.
Explicit missing/unverified state: when a receipt is missing or fails verification, the response now surfaces that explicitly (e.g. a distinct status/field indicating unverified payment) rather than silently reporting the same shape as a verified paid success. This prevents a caller from mistaking an unverified payment for a confirmed one.
Cross-runtime consistency: verified this behavior is aligned across Express, Vercel, browser, and MCP contexts — the receipt/explorer-URL exposure and the missing/unverified distinction behave the same regardless of which runtime is serving the request.
x402 settlement semantics preserved: no change to the underlying settlement verification logic itself — this PR only changes what's exposed in the tool's output, not how payment is verified.
Tests/docs: added coverage asserting the receipt object and explorer URL are present on verified paid responses, and that the explicit unverified/missing state is returned (not a false paid-success shape) when applicable. Updated relevant documentation describing the new response fields.
Why this approach

Surfacing the actual receipt object (rather than just amount/network) gives callers something independently verifiable instead of asking them to trust the server's summary. Making "missing/unverified" an explicit, distinct output state — rather than silently omitting the receipt field while still returning a success-shaped response — closes a gap where a caller could mistakenly treat an unverified payment as settled.

Testing
Unit/integration test: verified paid tool call returns response including receipt object and explorer URL.
Unit/integration test: simulated missing/failed receipt verification returns the explicit unverified state, not a paid-success shape.
Verified consistent behavior across Express, Vercel, browser, and MCP invocation paths.
Confirmed no change to underlying x402 settlement verification logic — existing settlement tests still pass unchanged.

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[AbuJulaybeeb]

kindly fetch upstream, rebase branch ad push

[AGWAM001]

ready for merge