Closes #60162.

MCP servers whose tool `inputSchema` uses `$ref`/`$defs` (e.g., Notion
MCP v2.x, and any server using Zod/Pydantic-generated schemas) are
silently rejected with:

```
ERROR Schema cannot be made compatible because it contains "$ref"
```

The affected tools are dropped from the agent panel — the user never
sees them and there is no user-visible error.

## Root cause

`adapt_to_json_schema_subset` rejects any schema containing `$ref` via
`UNSUPPORTED_KEYS`:

```rust
const UNSUPPORTED_KEYS: [&str; 4] = ["if", "then", "else", "$ref"];
```

This is hit by every provider that uses `JsonSchemaSubset` format
(Google Gemini, xAI Grok, OpenAI-compatible proxies, Vercel AI Gateway,
Copilot Chat for Google/xAI vendors, OpenRouter for gemini/grok models).
Providers using `JsonSchema` (Anthropic direct, OpenAI direct) don't hit
this check — `$ref` is passed through to the API, which may or may not
handle it correctly.

This is not an edge case. Every modern MCP server using Zod
(TypeScript), Pydantic (Python), or JSON Schema with shared definitions
generates `$ref`/`$defs` in tool schemas.

## Fix

Add a `resolve_refs` step in `adapt_schema_to_format` that dereferences
all `$ref` pointers using the document's own `$defs` (or legacy
`definitions`) map, making the schema self-contained before
format-specific processing. Applied at the entry point so both
`JsonSchema` and `JsonSchemaSubset` formats benefit.

**Scope note:** previously `JsonSchema` providers (Anthropic direct,
OpenAI direct) received the raw `$ref`/`$defs` and were expected to
handle it themselves — which most do not. After this change, both paths
receive a self-contained schema with refs inlined. This is intentional:
it fixes the same root cause for both paths and avoids provider-specific
behavior divergence.

Supported `$ref` forms:
- `#/$defs/<name>` (JSON Schema draft 2019-09+)
- `#/definitions/<name>` (draft 4-7 legacy)

Edge cases:
- **Nested `$ref`** (definition references another definition): resolved
recursively.
- **Sibling properties alongside `$ref`** (e.g. `{ "$ref": "...",
"description": "..." }`, legal under draft 2019-09+): merged onto the
resolved definition, with siblings overriding the definition's keys.
- **Cyclic references** (A → B → A, or self-referential schemas like a
Tree node): replaced with an empty schema `{}` ("any JSON value"). The
tool still works, just without type info for that recursive field.
- **Unsupported `$ref` forms** (e.g., external URLs): returns an error
with a clear message.
- **Missing definition target**: returns an error naming the missing
ref.

## Testing

Added 10 unit tests in `crates/language_model_core/src/tool_schema.rs`
that cover the patterns produced by Zod/Pydantic-generated MCP schemas:

- `test_refs_are_resolved_via_adapt_schema_to_format` — basic `$ref` →
`$defs` resolution
- `test_refs_in_defs_are_resolved` — nested `$ref` (definition
references another definition)
- `test_refs_in_array_items_are_resolved` — `$ref` inside `array.items`
- `test_legacy_definitions_prefix_is_supported` — old
`#/definitions/<name>` prefix
- `test_schema_without_defs_is_unchanged` — schemas with no `$defs` are
unaffected
- `test_refs_fail_for_unsupported_prefix` — external URL refs error
clearly
- `test_refs_fail_for_missing_definition` — missing target errors
clearly
- `test_cyclic_refs_are_replaced_with_empty_schema` — A → B → A cycle
replaced with `{}`
- `test_self_referential_ref_is_replaced_with_empty_schema` — Tree-like
self-ref replaced with `{}`
- `test_ref_sibling_properties_are_preserved` — sibling properties
alongside `$ref` are merged onto the resolved definition

Existing tests (which call `adapt_to_json_schema_subset` and
`preprocess_json_schema` directly) are unaffected — the fix is additive
at the `adapt_schema_to_format` level.

## Disclosure

I used an LLM to help draft the implementation and tests. I reviewed and
understand the change — it adds one new function (`resolve_refs`) with
two helpers (`parse_ref`, `resolve_refs_recursive`), plus unit tests.

Release Notes:

- Fixed MCP tools with `$ref`/`$defs` in their `inputSchema` being
silently rejected by providers using the JSON Schema Subset format
(Google Gemini, xAI Grok, OpenAI-compatible proxies, etc.). Tools from
servers like Notion MCP v2.x, and any server using Zod or
Pydantic-generated schemas, now work correctly.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>