feat(mcp): switch ask_hexclave model from gemini to glm 5.2 (#1677)

## Summary

Switches the `ask_hexclave` MCP tool's model from Gemini 3.5 Flash to
GLM 5.2 by changing the speed parameter from `"fast"` to `"slow"` in the
query to the AI backend.

The model selection matrix maps `smart`/`slow`/`unauthenticated` →
`z-ai/glm-5.2` (previously `smart`/`fast`/`unauthenticated` →
`google/gemini-3.5-flash`).

Link to Devin session:
https://app.devin.ai/sessions/f710490916754099916f9899c2700a5c
Requested by: @mantrakp04

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Switched the `ask_hexclave` MCP tool from Gemini to GLM by updating the
model selection matrix. Unauthenticated smart/fast now routes to
`z-ai/glm-5:nitro`, unauthenticated smartest/fast to
`z-ai/glm-5.2:nitro`, and `dumb/slow/authenticated` uses
`z-ai/glm-4.5-air` (removed `:free`).

<sup>Written for commit 6a51ba6358.
Summary will update on new commits.</sup>

<a
href="https://cubic.dev/pr/hexclave/hexclave/pull/1677?utm_source=github"
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<!-- End of auto-generated description by cubic. -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Adjusted AI request behavior to favor more deliberate responses, which
may improve result quality and consistency.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: mantra <mantra@stack-auth.com>

apps/backend/src/lib/ai/models.ts

@@ -34,7 +34,7 @@ const MODEL_SELECTION_MATRIX: Record<
     },
     fast: {
       authenticated: { modelId: "openai/gpt-5.5" },
-      unauthenticated: { modelId: "google/gemini-3.5-flash" },
+      unauthenticated: { modelId: "z-ai/glm-5:nitro" },
     },
   },
   smartest: {
@@ -44,7 +44,7 @@ const MODEL_SELECTION_MATRIX: Record<
     },
     fast: {
       authenticated: { modelId: "openai/gpt-5.5" },
-      unauthenticated: { modelId: "google/gemini-3.5-flash" },
+      unauthenticated: { modelId: "z-ai/glm-5.2:nitro" },
     },
   },
 };