feat(provider): add MiniMax M3 model and prune deprecated M2.x variants

[octo-patch]

Summary

Updates the MiniMax provider model catalog in crates/forge_repo/src/provider/provider.json:

• Add MiniMax-M3 as the new top-of-list (default) entry — the latest generation MiniMax model with enhanced reasoning, function calling, and an extended (1M) context window.
• Keep MiniMax-M2.7 and MiniMax-M2.7-highspeed as legacy options for users still on the previous generation.
• Remove the deprecated entries: MiniMax-M2, MiniMax-M2.1, MiniMax-M2.1-highspeed, MiniMax-M2.5, MiniMax-M2.5-highspeed. These are no longer recommended for new sessions; the M2.7 line covers the same use cases at parity or better quality.

This follows the same minimal-edit pattern used for prior model catalog updates (e.g. #2727, #2701, #2597, #2557). Other knobs — base URL (api.minimax.io), MINIMAX_API_KEY env var, the OpenRouter SetMinimaxParams transformer, and OpenCode model routing — are intentionally untouched.

Why

MiniMax-M3 is the current production model at MiniMax and outperforms the M2.x line on coding/agentic benchmarks. Surfacing it as the default lets new users pick it up automatically; pruning the older entries keeps the picker focused on the models that are still recommended.

Changes

• crates/forge_repo/src/provider/provider.json — only the id: "minimax" models array is touched. 9 insertions / 49 deletions, all data-only (no schema changes).

Test plan

• cargo check -p forge_repo passes (data-only edit; existing ProviderConfig schema is unchanged).
• cargo test -p forge_repo — test_load_provider_configs still loads the catalog; no minimax-specific assertions exist that depend on the removed model IDs.
• Manual: forge lists MiniMax-M3 first under the MiniMax provider; selecting it routes through the existing Anthropic-compatible endpoint without code changes.

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[amitksingh1490]

closing in favour of #3434