fix: validate Bazaar schema formats

[epistemedeus]

Description

validateDiscoveryExtension() compiled Bazaar schemas without standard JSON
Schema format handlers. AJV therefore ignored declarations such as
format: "uri" and format: "date-time", accepted invalid values, and printed
warnings while validating otherwise machine-readable discovery data.

This patch registers ajv-formats on the existing AJV instance and adds invalid
and valid URI/date-time cases. It does not change the Bazaar schema shape or
payment behavior.

Most of this patch was generated with Codex. I reviewed the source and final
diff, reproduced the defect with the published package, removed unrelated
lockfile changes, and ran the checks below.

Tests

• pnpm --filter @x402/core build
• pnpm --filter @x402/extensions test (386 passed)
• pnpm --filter @x402/extensions lint:check
• pnpm --filter @x402/extensions format:check
• pnpm --filter @x402/extensions build
• frozen-lockfile install

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commit is signed
• I added a changelog fragment for the user-facing fix

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1