fix(provider): add glm-5.3 to zai_coding provider

[aditzel]

Summary

GLM-5.3 was released on 2026-08-14 and is available to all GLM Coding Plan subscribers (announcement, docs), but it is missing from the hardcoded zai_coding model catalog. This PR adds it.

Changes

• crates/forge_repo/src/provider/provider.json — Added glm-5.3 as the first model under the zai_coding provider, mirroring the existing glm-5.2 entry schema:
  • context_length: 1048576 (1M, per official docs)
  • tools_supported, supports_parallel_tool_calls, supports_reasoning: true
  • input_modalities: ["text"] (GLM-5.3 is text-only)
  • Description notes the 50% coding gain over GLM-5.2, 1M context, 128K max output, and always-on reasoning (low/high/max)

Why only zai_coding and not zai

The GLM Coding Plan endpoint (https://api.z.ai/api/coding/paas/v4) serves GLM-5.3 to all plan tiers now. The general Model API endpoint still lists GLM-5.3 as "available soon" (docs), so the entry is intentionally not added to the zai provider yet.

Notes for reviewers

• z.ai routes requests for glm-5.2/glm-5.1 to GLM-5.3 server-side (devpack docs), so existing users are unaffected; this just makes the real model ID selectable.
• GLM-5.3 does not support disabling reasoning — Forge's thinking transformation for zai providers already always maps reasoning.enabled to thinking: {type: enabled}, which matches the required payload.
• Purely additive JSON change; validated parse + schema parity with the glm-5.2 entry. No existing tests reference zai model lists, so no test changes were needed.

Verification

• provider.json parses cleanly and the new entry uses the identical key set as glm-5.2
• Confirmed GLM-5.3 specs against official docs (1M context, 128K output, text-only, always-on reasoning)
• Confirmed no zai-specific model assertions in provider_repo.rs or integration tests

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