chore(ci): add duplicate anchor ID detection for composed MDX pages

[marythought]

Problem

When multiple MDX files are imported into the same page (e.g. code_samples/tdf/encrypt_options.mdx imported into docs/sdks/tdf.mdx), headers from both files generate anchor IDs on the same rendered page. If two headers produce the same slug, TOC links jump to the wrong section.

This just bit us — ### Assertions in encrypt_options.mdx collided with ## Assertions in tdf.mdx, along with ### Data Attributes and ### Metadata.

Proposed solution

Add a script (e.g. src/openapi/check-duplicate-anchors.ts or similar) that:

1. Parses each docs MDX file for import statements referencing code_samples/
2. Collects all markdown headers from the doc file and its imports
3. Slugifies them the same way Docusaurus does
4. Fails if any duplicates are found

Run it in CI alongside check-vendored-yaml.

Context

Fixed in #268 by renaming encrypt option headers to use SDK option names (WithAssertions, WithDataAttributes, WithMetadata). Convention documented in AGENTS.md — but a CI check would catch future regressions.