chore: retire mark3labs/mcp-go dependency (#28061)

## Stack Context

PR 6 of 6 in a stack that migrates every Coder MCP surface from the
archived `github.com/mark3labs/mcp-go` library to the official
`github.com/modelcontextprotocol/go-sdk` v1.7.0.

Stack: #28056 -> #28057 -> #28058 -> #28059 -> #28060 -> #28061

## Why

With every production surface migrated, this PR removes the mark3labs
dependency entirely and converts the remaining test fixtures.

- Migrates the remaining mark3labs test fixtures (coderd MCP e2e tests,
chatd fixtures, mcpclient fixtures, and the Force On MCP policy tests)
to official stateless SDK servers.
- Removes `github.com/mark3labs/mcp-go` from `go.mod` and drops the
corresponding dependabot ignore entry. Zero references remain repo-wide.
- Updates the MCP docs for the 2026-07-28 protocol: stateless Streamable
HTTP behavior, the supported 2024-11-05 through 2026-07-28 protocol
range, and explicit non-features (resources, prompts, structured output,
elicitation, MCP Tasks).
- The e2e ping assertion is removed because MCP 2026-07-28 removed the
ping method.

> Mux created this PR on Mike's behalf.

go.sum

@@ -860,8 +860,6 @@ github.com/makeworld-the-better-one/dither/v2 v2.4.0 h1:Az/dYXiTcwcRSe59Hzw4RI1r
 github.com/makeworld-the-better-one/dither/v2 v2.4.0/go.mod h1:VBtN8DXO7SNtyGmLiGA7IsFeKrBkQPze1/iAeM95arc=
 github.com/marekm4/color-extractor v1.2.1 h1:3Zb2tQsn6bITZ8MBVhc33Qn1k5/SEuZ18mrXGUqIwn0=
 github.com/marekm4/color-extractor v1.2.1/go.mod h1:90VjmiHI6M8ez9eYUaXLdcKnS+BAOp7w+NpwBdkJmpA=
-github.com/mark3labs/mcp-go v0.38.0 h1:E5tmJiIXkhwlV0pLAwAT0O5ZjUZSISE/2Jxg+6vpq4I=
-github.com/mark3labs/mcp-go v0.38.0/go.mod h1:T7tUa2jO6MavG+3P25Oy/jR7iCeJPHImCZHRymCn39g=
 github.com/mattn/go-colorable v0.1.2/go.mod h1:U0ppj6V5qS13XJ6of8GYAs25YV2eR4EVcfRqFIhoBtE=
 github.com/mattn/go-colorable v0.1.9/go.mod h1:u6P/XSegPjTcexA+o6vUJrdnUu04hMope9wVRipJSqc=
 github.com/mattn/go-colorable v0.1.12/go.mod h1:u5H1YNBxpqRaxsYJYSkiCWKzEfiAb1Gb520KVy5xxl4=