fix(schema): update checkout_mandate pattern to support dSD-JWT chain (#599)

[Harman-2]

Description

Updates the regex validation pattern for the checkout_mandate property inside the ap2_mandate.json schema. The previous regex constraint strictly required characters immediately following a tilde (~), which caused schema validation to fail when encountering the trailing tilde of compact SD-JWT strings or multi-hop ~~ delegate-SD-JWT chains emitted by the reference implementation. Loosening the pattern group modifier from + to * allows both empty segments and trailing tildes while maintaining the required character set validation.

Category (Required)

• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)

Related Issues

Fixes #599

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

Verified regex behavior variation via python environment simulation:

# Before Fix Pattern Verification (Fails on dSD-JWT wire format strings)
Before fix match result: None

# After Fix Pattern Verification (Successfully parses trailing tildes and multi-hop '~~' chains)
After fix match result: <re.Match object; span=(0, 62), match='eyJhbGciOiJFUzI1NiJ9.payload.sig~~disclosure1~disclosure2~'>

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[damaz91]

Hello @Harman-2, can you please sign the CLA (https://cla.developers.google.com/ )? It is a required step to contribute to UCP? Thanks in advance!

[Harman-2]

@google-cla recheck

[Harman-2]

@damaz91 Thanks for heads up! The CLA has been signed.