fix(prompts): fetch gateway-backed prompt content in server listing

[Ewertonslv]

🔗 Related Issue

Closes #6440

📝 Summary

GET /v1/servers/{server_id}/prompts returned "template": "" for every
auto-discovered (gateway-backed) prompt. Custom prompts returned their
content correctly; getting a gateway-backed prompt's actual content required
a separate GET /v1/prompts/{id} call per prompt.

Root cause

Gateway-backed prompts are synced into the catalog as metadata only — the
MCP prompts/list protocol call doesn't return content, so template is
stored as "" at sync time. The single-prompt endpoint (GET /v1/prompts/{id}
→ get_prompt()) already detects this via _should_fetch_gateway_prompt() and
fetches the content live from upstream via _fetch_gateway_prompt_result().
list_server_prompts() in prompt_service.py never applied that same check,
so it always returned the empty stored value.

Fix

After the listing query, identify gateway-backed prompts with
_should_fetch_gateway_prompt() (existing helper, unchanged) and fetch their
live content concurrently via asyncio.gather(...), applying the fetched text
onto the already-built PromptRead objects. A per-prompt fetch failure is
caught, logged, and leaves that prompt's template as the empty string it
already was — one unreachable gateway doesn't fail the whole listing.

Tests (before → after)

• test_gateway_backed_prompt_template_fetched_live: a gateway-backed prompt
  (empty template) and a local custom prompt (populated template) in the same
  listing — asserts the gateway-backed one gets its live content, the local one
  is untouched. Without the fix, _fetch_gateway_prompt_result is never
  called (Awaited 0 times) and the read stays empty; with the fix, both
  assertions hold. Verified RED→GREEN via git stash.
• test_gateway_backed_prompt_fetch_failure_leaves_template_empty: an
  unreachable gateway raises PromptError — the listing still returns the
  prompt, with template left as "" (graceful degradation, no 500).

pytest tests/unit/mcpgateway/services/test_prompt_service.py — full file
green (all tests, not just the two new ones).

📏 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