feat(embedded): add link delegation extension

[westeezy]

Description

Embedded Checkout can present links from the business (e.g., privacy policy, terms of service). By default, the checkout handles these internally. Link delegation lets the host claim control: when ec_delegate=link.open is set, the Embedded Checkout MUST send ec.link.open_request on buyer link activation, and the host MUST present the content or respond with a link_rejected error.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

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Is this a Breaking Change or Removal?

If you checked "Breaking change" above, or if you are removing any schema
files or fields:

• I have added ! to my PR title (e.g., feat!: remove field).
• I have added justification below.

## Breaking Changes / Removal Justification

(Please provide a detailed technical and strategic rationale here for why this
breaking change or removal is necessary.)

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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
• Any dependent changes have been merged and published in downstream modules