feat: add service metadata on resource to all 3 SDKs

[aayushijain21]

Description

Adds three optional service-metadata fields — serviceName, tags, iconUrl — to the top-level resource object on a PaymentRequired response, and formalizes them in the bazaar extension spec. Facilitators use these to enrich Bazaar search results with a provider-published name, topical tags, and an icon, with no admin step.

The CDP facilitator will parse these fields defensively from 402 responses. This PR formalizes the convention upstream so all three SDKs and the foundation spec agree on the shape and validation rules.

All changes are purely additive and backwards compatible:

• New fields are optional in TS / Python / Go (?:, | None = None, omitempty); existing servers produce byte-identical 402 bodies.
• New helpers and types are new exports — no existing function signature, field, or test was changed or removed.
• Spec change is a new section; existing tables and examples are untouched apart from one example gaining the three new keys.

Tests

New tests in each SDK cover the helpers (boundaries at 32/33 chars, tag truncation, data: / file: / javascript: / ftp: schemes, userinfo, IPv4 and IPv6 literals including [::1] and [2001:db8::1], mixed-case localhost, control characters \x00 / \n / \x7f, relative paths) plus end-to-end extraction cases for the all-valid path and the all-invalid soft-drop path.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits
• I added a changelog fragment for user-facing changes (docs-only changes can skip)

[vercel]

@aayushijain21 is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[CarsonRoscoe]

Please update the x402-specification-v2.md file:

1. The **5.1 PaymentRequired Schema** section's json example to include these fields (lines 74-99)
2. The ResourceInfo table (lines 125-133) to include these new fields as optional at the spec level