Make mode in elicitation backwards compatible (modelcontextprotocol#1863)

* Make mode in elicitation backwards compatible

* Make mode optional in ElicitRequestFormParams for backwards compatibility

For backwards compatibility with servers that don't include the mode field
in form-based elicitation requests, mode is now optional and defaults to 'form'.

This aligns with the spec language stating that an empty elicitation capability
object is equivalent to declaring support for form mode only.

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Co-Authored-By: Claude <[email protected]>

* Update elicitation docs to reflect optional mode field

Clarify that servers MAY omit the mode field for form mode elicitation
requests, and clients MUST treat such requests as form mode.

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Co-Authored-By: Claude <[email protected]>

---------

Co-authored-by: Claude <[email protected]>

Author: pcarleton

docs/specification/draft/client/elicitation.mdx

@@ -79,21 +79,23 @@ To request information from a user, servers send an `elicitation/create` request
 
 All elicitation requests **MUST** include the following parameters:
 
-| Name      | Type   | Options       | Description                                                        |
-| --------- | ------ | ------------- | ------------------------------------------------------------------ |
-| `mode`    | string | `form`, `url` | The mode of the elicitation.                                       |
-| `message` | string |               | A human-readable message explaining why the interaction is needed. |
+| Name      | Type   | Options       | Description                                                                            |
+| --------- | ------ | ------------- | -------------------------------------------------------------------------------------- |
+| `mode`    | string | `form`, `url` | The mode of the elicitation. Optional for form mode (defaults to `"form"` if omitted). |
+| `message` | string |               | A human-readable message explaining why the interaction is needed.                     |
 
 The `mode` parameter specifies the type of elicitation:
 
 - `"form"`: In-band structured data collection with optional schema validation. Data is exposed to the client.
 - `"url"`: Out-of-band interaction via URL navigation. Data (other than the URL itself) is **not** exposed to the client.
 
+For backwards compatibility, servers **MAY** omit the `mode` field for form mode elicitation requests. Clients **MUST** treat requests without a `mode` field as form mode.
+
 ### Form Mode Elicitation Requests
 
 Form mode elicitation allows servers to collect structured data directly through the MCP client.
 
-Form mode elicitation requests **MUST** specify `mode: "form"` and include these additional parameters:
+Form mode elicitation requests **MUST** either specify `mode: "form"` or omit the `mode` field, and include these additional parameters:
 
 | Name              | Type   | Description                                                    |
 | ----------------- | ------ | -------------------------------------------------------------- |