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

[Ewertonslv]

🔗 Related Issue

Closes #5451

📝 Summary

In direct_proxy gateway mode, ResourceService.read_resource fetches the
resource live from the upstream and builds the final TextResourceContents /
BlobResourceContents. Execution then fell 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 raised
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 did 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.

pytest tests/unit/mcpgateway/services/test_resource_service.py passes locally
(full file green, including module doctests via --doctest-modules).

📏 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