Fix cross-realm linksTo MIME type handling for file/image defs

[lucaslyl]

linear: https://linear.app/cardstack/issue/CS-10730/card-thumbnail-linksto-imagedeffiledef-fails-to-save-for-cross-realm

Previously, cross-realm linksTo requests for file-def and image-def could send an incorrect Accept header and fail with 415 Unsupported Media Type.(from console)

Changes:

1. In loadLinks, use expectsCard as the primary signal for cross-realm links.
2. If expectsCard is true, fetch with card MIME type.
3. Otherwise, fetch with file-meta MIME type.
4. Remove the card-only isSingleCardDocument assertion from this mixed cross-realm path to avoid false failures on valid file-meta responses.

Result:

Cross-realm file-def/image-def links resolve correctly.
Eliminates 415 errors caused by wrong media type negotiation.

Screen.Recording.2026-04-14.at.6.56.06.PM.mov

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Default unknown cross-realm links to card MIME

When relationship.data.type is omitted (a valid/stale case already handled above), expectsCard stays false unless the type is explicitly card. In this cross-realm branch that means we now send Accept: application/vnd.card.file-meta+json by default, while the in-realm branch still defaults unknown types to card lookup. As a result, cross-realm card links with missing data.type can negotiate the wrong representation (or fail with media-type errors), causing linked card relationships to stop resolving correctly.

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[github-actions]

Realm Server Test Results

  1 files  ±0    1 suites  ±0   14m 8s ⏱️ +21s
845 tests +1  845 ✅ +1  0 💤 ±0  0 ❌ ±0 
916 runs  +1  916 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit e06f743. ± Comparison against base commit 72d1f10.

[github-actions]

Host Test Results

    1 files  ±0      1 suites  ±0   2h 29m 37s ⏱️ - 4m 5s
2 194 tests ±0  2 151 ✅  - 28  15 💤 ±0   0 ❌ ± 0  28 🔥 +28 
2 194 runs  ±0  2 123 ✅  - 56  15 💤 ±0  28 ❌ +28  28 🔥 +28 

For more details on these errors, see this check.

Results for commit e06f743. ± Comparison against base commit 72d1f10.