MCP providers crash when inputSchema contains an unsupported parameter type

### Self Checks

- [x] I have read the [Contributing Guide](https://github.com/langgenius/dify/blob/main/CONTRIBUTING.md) and [Language Policy](https://github.com/langgenius/dify/issues/1542).
- [x] This is only for a bug report; questions belong in Discussions.
- [x] I have searched existing issues, including closed ones.
- [x] I am submitting this report in English.
- [x] I have kept the required bug-report fields and filled them in.

### Dify version

`main` at `e35187f37626a1d3a7af5bedbebc19d40cf20cb5` (source checkout)

### Cloud or Self Hosted

Self Hosted (Source)

### Steps to reproduce

This is a source-level reproduction for an MCP schema supplied by a third-party server.

1. Have an MCP tool expose an `inputSchema` containing a property whose JSON Schema `type` is not supported by Dify, for example:

```json
{
"type": "object",
"properties": {
"query": {
"type": "text"
}
}
}
```

The same problem occurs with values such as `"String"` or a type list such as `["bogus", "null"]`.

2. Persist/register that MCP provider, then load its tools, update its credentials, or invoke one of its tools.
3. `convert_mcp_schema_to_parameter` calls `resolve_property_type`, which returns the unsupported raw type, and then passes it to `ToolParameter.ToolParameterType(prop_type)`.

The conversion raises:

```
ValueError: 'text' is not a valid ToolParameter.ToolParameterType
```

Because the third-party schema is persisted as supplied, subsequent provider loads hit the same failure again. A focused regression test reproducing this path is included in #38788.

### ✔️ Expected Behavior

An unsupported or malformed third-party MCP parameter type should not make the entire provider unloadable. Consistent with `resolve_property_type`'s existing fallback behavior for null, empty, cyclic, and over-depth schemas, unsupported JSON Schema type names should safely resolve to `string`. Standard JSON Schema types should remain unchanged.

### ❌ Actual Behavior

The unsupported raw type reaches the `ToolParameterType` enum constructor and raises an unhandled `ValueError`. This can break provider listing/loading, credential updates, and invocation for the affected MCP provider.

A proposed fix and regression tests are available in #38788.

Process note: I opened the implementation PR before noticing Dify's issue-first and assignment requirement. This issue documents the failure transparently; I am happy to pause or adjust the implementation until a maintainer confirms the direction and assigns the issue.

Start by reading convert_mcp_schema_to_parameter and resolve_property_type, then inspect the focused regression test referenced in #38788. Confirm that unsupported MCP schema types fall back safely while standard JSON Schema types remain unchanged, and run the regression test to verify provider loading no longer raises ValueError.

Written by the indexing model from the issue text.