[FEATURE]: Support explicit cost tiers for custom provider models

[richardsun0713]

Feature has not been suggested before.

• I searched for existing issues and found Local cost estimate uses the 200k price below a model's real context threshold #44224 and Incorrect session cost estimate for models with context tiered pricing #42910. Both cover incorrect selection of catalog tiers at runtime; this request adds the missing public config surface for custom provider models to supply those tiers.

Describe the enhancement you want to request

Custom provider models cannot configure arbitrary context-pricing tiers. This prevents exact local cost estimates for OpenAI-compatible gateways that charge a long-context rate above 272K input tokens.

OpenCode runtime cost models support arbitrary context tiers, but custom models in opencode.json expose only the legacy context_over_200k field.

Allow provider.*.models.*.cost.tiers in config, using the existing internal and models.dev shape:

{
  "cost": {
    "input": 2,
    "output": 10,
    "cache_read": 0.2,
    "cache_write": 2.5,
    "tiers": [
      {
        "tier": { "type": "context", "size": 272000 },
        "input": 4,
        "output": 15,
        "cache_read": 0.4,
        "cache_write": 5
      }
    ]
  }
}

Expected behavior:

1. Configured tiers are accepted and included in a custom model cost definition.
2. A configured tier replaces a catalog tier with the same type and size.
3. Unrelated catalog tiers are preserved.
4. Cost estimation uses the base rate at or below 272K tokens and the configured tier above it.

This complements #44223 and #44292, which address runtime handling of catalog tiers but intentionally do not add arbitrary tiers to custom configuration.

[github-actions]

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What needs to be fixed:

• Feature requests should use the feature request template, which requires a title starting with [FEATURE]: and a verification checkbox confirming you've searched for existing issues
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Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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This issue might be a duplicate of existing issues. Please check:

• Local cost estimate uses the 200k price below a model's real context threshold #44224: Local cost estimate uses wrong price tier for requests between 200k and a model's real context threshold (same underlying context_over_200k / tiers gap)
• Incorrect session cost estimate for models with context tiered pricing #42910: Incorrect session cost estimate for models with context tiered pricing

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[github-actions]

Thanks for updating the issue! It now follows the contributing guidelines. We'll take a look.