…#6853)

## Summary

- `resolveMediaRelativeToCloud` previously treated every `image`-typed
value as a media-library path — wrapping it with the cloud assets prefix
on read. That corrupts any value that's already an absolute URL
(external link, `data:` URL, etc.), because the prefix gets concatenated
directly in front of the original URL with no separator.
- Adds a guard for absolute URLs (any RFC 3986 scheme, plus
protocol-relative `//`) so they round-trip identically in both
directions.
- Adds a self-heal: if stripping the configured media root from a stored
value reveals an absolute URL underneath (the shape of a
previously-corrupted round-trip), return the URL directly instead of
re-wrapping. Documents already on disk in the broken shape transparently
recover on next save.
- The save-side resolver already gated rewriting on a cloud-URL pattern
match, so no change there beyond inheriting cleaner inputs.

## Verification

Drove the full `parseMDX` → resolver → `serializeMDX` pipeline against a
rich-text body containing both a media-library src and an external URL,
on a feature branch with a staging prefix active:

| Phase | Local `src` | External `src` |
|---|---|---|
| Stored MDX | `/uploads/library/local.png` |
`https://cdn.example.com/external/image.png` |
| After read (editor sees) |
`https://<assetsHost>/<clientId>/__staging/<branch>/__file/library/local.png`
| `https://cdn.example.com/external/image.png` |
| After write (re-serialized MDX) | `/uploads/library/local.png` |
`https://cdn.example.com/external/image.png` |

Self-heal verified end-to-end: a corrupted stored src of the form
`/uploads<absolute-url>` is read as the clean absolute URL and
serialized back to disk in that form.

Confirmed the same demo fails against the unpatched resolver — the
external URL gets clobbered with a leading
`https://<assetsHost>/<clientId>/__staging/<branch>/__file` and then
mangled further on save.

## Test plan

- [x] `pnpm --filter @tinacms/graphql test` — 516/516 passing across 50
files
- [x] New `media-utils.test.ts` cases:
- [x] `it.each` over `https://`, `http://`, `//`, `data:`, `blob:`,
`file:` — identity in both directions on a feature branch with active
staging prefix
- [x] Mixed array — relative entries get the staging prefix, absolute
entries pass through untouched
- [x] `it.each` self-heal cases — `/uploads<url>` shapes for `https://`,
`http://`, `//`, `data:` recover to the underlying URL on read
- [x] Array self-heal — mixed clean + corrupted entries are all
normalized
- [x] New `media-rich-text.test.ts` end-to-end cases:
- [x] Rich-text body with a mix of local + external `imageEmbed` srcs —
full parse/serialize round-trip identity
  - [x] Protocol-relative and `http://` srcs — same identity guarantee
- [x] Corrupted src — read self-heals, save writes clean form back to
disk
- [ ] Smoke test in a branch-enabled site: open a document containing an
`image`-typed field whose stored value is an external URL, save without
touching it, confirm the stored value is unchanged

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>