DOCS-1788 - Document AgentCore Gateway setup for MCP server

[kimsauce]

Purpose of this pull request

Customers connecting the Sumo Logic MCP server through an AWS Bedrock AgentCore Gateway (to aggregate multiple MCP servers behind one endpoint) hit an opaque Error parsing ClientCredentials response error, because the gateway target's OAuth2 credential provider config needs grantType: CLIENT_CREDENTIALS explicitly set — an AWS requirement not obvious from our own OAuth setup docs.

Adds one sentence to the OAuth section of docs/api/mcp-server.md noting that the MCP server works behind gateway aggregators like AWS Bedrock AgentCore Gateway, that these should be configured with the Client Credentials flow, and that gateways may need grantType: CLIENT_CREDENTIALS set explicitly to avoid the error above.

Source: internal Slack thread (INFOR customer, $3.2M ARR); fix confirmed working by Lokent Banswani; requested by Vijit Singhal.

Select the type of change

• Minor Changes - Typos, formatting, slight revisions
• Update Content - Revisions, updating sections
• New Content - New features, sections, pages, tutorials
• Site and Tools - .clabot, version updates, maintenance, dependencies, new packages for the site (Docusaurus, Gatsby, React, etc.)

Ticket (if applicable)

https://sumologic.atlassian.net/browse/DOCS-1788

[kdwink]

This small additional note looks good to me.

We could potentially remove this last part of the last sentenc:

"though Sumo Logic's own OAuth setup doesn't need it named explicitly"

In our setup user do select the client type as "client credentials" and behind the scenes that configures a CLIENT_CREDENTIALS grant type. So while true, in a sense, maybe not relevant? Not a big deal either way.

[kimsauce]

This small additional note looks good to me.

We could potentially remove this last part of the last sentenc:

"though Sumo Logic's own OAuth setup doesn't need it named explicitly"

In our setup user do select the client type as "client credentials" and behind the scenes that configures a CLIENT_CREDENTIALS grant type. So while true, in a sense, maybe not relevant? Not a big deal either way.

Thanks, @kdwink ! Just applied this change.