fix: correct Copilot GPT-5.2 token limits

[dlukt]

What does this PR do?

Fixes #11086

GitHub Copilot’s gpt-5.2 / gpt-5.2-codex models are currently ingested from models.dev with incorrect limit values, which makes OpenCode think the context window is much smaller than it really is. Since OpenCode uses model.limit.context / model.limit.output for both the context meter and auto-compaction overflow checks, this shows an inflated % used and triggers compaction far too early (e.g. ~90–100k tokens). (Refs #11086, duplicate #12247.)

This PR fixes that by overriding the token limits during models.dev -> Provider.Model conversion, but only for providerID starting with github-copilot and only for model IDs gpt-5.2 and gpt-5.2-codex. The override sets:

• context: 400000
• input: 272000
• output: 128000

This works because every downstream consumer (UI usage, overflow/compaction thresholds, max output calculations) reads from Provider.Model.limit. Correcting the limits at ingestion time fixes behavior without changing session logic, and the scope is intentionally narrow to avoid impacting other Copilot models.

How did you verify your code works?

• Added a regression test that asserts the override is applied for both github-copilot/gpt-5.2 and github-copilot/gpt-5.2-codex.
• Ran cd packages/opencode && bun test test/provider/provider.test.ts.

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