docs(providers): document owner_signup_limit on Provider

[sidharth0612]

Documents the owner_signup_limit field that GET /v0/providers and GET /v0/providers/{provider_id} publish for listed providers, shipped in agentmail-to/agentmail-api#1315.

• owner_signup_limit (optional integer): the provider's cap on how many accounts the caller's organization may sign up at it.
• Omitted when the provider sets no limit. 0 means the provider has paused new sign-ups while existing accounts keep signing in.

Updated in both the Fern definition (providers.yml) and the OpenAPI export, in step, as prior provider/API-key docs commits did.

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✨ API Changes

# API Changelog n/a vs. n/a


## API Changes

### GET /v0/providers
-  added the optional property `providers/items/owner_signup_limit` to the response with the `200` status


### GET /v0/providers/search
-  added the optional property `providers/items/owner_signup_limit` to the response with the `200` status


### GET /v0/providers/{provider_id}
-  added the optional property `owner_signup_limit` to the response with the `200` status


### GET /v0/providers/{provider_id}/accounts
-  added the optional property `provider/owner_signup_limit` to the response with the `200` status

💡 Download api-changelog-diff artifact or tag @fern Writer in #github-prs for changelog.

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