fix(resources): skip post-fetch resolution for direct_proxy reads

[Ewertonslv]

🔗 Related Issue

Closes #5451

Fresh PR replacing #5463. That PR was closed on 2026-08-31 during the housekeeping pass on long-open contributions. GitHub refused to re-open it after the rebase (a force-push makes a closed PR non-reopenable), so this is the "or open a fresh one" path @jonpspri suggested. The change itself is unchanged — only rebased.

📝 Summary

In direct_proxy gateway mode, ResourceService.read_resource fetches the
resource live from the upstream and builds the final TextResourceContents /
BlobResourceContents. Execution then falls through into the shared post-fetch
RESOLVE CONTENT block, whose first branch runs getattr(content, "id")
unconditionally. Those MCP-compliant models (mcpgateway/common/models.py) have
no id field, so every direct_proxy text or blob read raises
AttributeError: 'TextResourceContents' object has no attribute 'id'. The
transport layer catches the exception and returns empty content, so the client
silently receives text="" instead of the fetched payload.

Root cause

The direct_proxy branch never marked its content as already-resolved — the
# Skip the rest of the DB lookup logic comment does not actually skip anything.

Fix

Track whether content came from the direct_proxy branch (direct_proxy_read) and
skip the metadata→content resolution for it. The gateway access check, the
resource access check, and the post-fetch plugin hooks all still run — only the
id-based invoke_resource re-resolution is bypassed, since the payload is final.

Tests (before → after)

The existing happy-path direct_proxy tests masked the bug by monkey-patching
id-bearing subclasses over the real models, so production classes never took the
failing path. This PR adds two regression tests exercising the real
TextResourceContents / BlobResourceContents:

• Without the fix: both raise AttributeError at the getattr(content, "id") call.
• With the fix: text read returns the live text, image/png blob read returns its
  blob, and invoke_resource is asserted not re-awaited.

🔄 Rebase notes (2026-08-31)

Rebased onto current main (ffcc0822). One conflict, resolved by keeping both
sides: main grew the _set_gateway_content helper immediately above the
if isinstance(content, ...) branch. The helper is untouched — the
direct_proxy_read guard simply sits in front of it as
if direct_proxy_read: ... elif isinstance(...).

Re-verified against today's main, since a lot landed in the six weeks the PR sat:

• The bug is still present. The RESOLVE CONTENT block still calls
  getattr(content, "id") unconditionally, and the MCP-compliant models still have
  no id.
• The regression proof still holds. With the production hunk reverted to main,
  both new tests fail with
  AttributeError: 'TextResourceContents' object has no attribute 'id'; with the
  hunk applied, both pass.
• pytest tests/unit/mcpgateway/services/test_resource_service.py — 309 passed.
• ruff check, ruff format --check and black --check clean on both touched files.

Also: #5451 is no longer labeled triage (it is bug / COULD now), which was the
concern I flagged in my comment on the original PR.

📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked in separate issues/PRs
• Tests are included with the code they validate
• If AI-assisted, I understand and can explain the generated changes

🏷️ Type of Change

• Bug fix