Fix Streamable handling to support JSON-only responses

[devcrocod]

quick fix #308

How Has This Been Tested?

locally

Breaking Changes

No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[Copilot]

Pull Request Overview

This PR fixes the Streamable handling to properly support JSON-only responses, addressing issue #308. The change improves compatibility with servers that don't support Server-Sent Events (SSE) by checking the response content type.

• Added content type checking for JSON responses in SSE error handling
• Enhanced logging to distinguish between method not allowed and JSON-only mode
• Graceful fallback when server returns JSON instead of SSE

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[Copilot]

[nitpick] Consider extracting the response object to a local variable to avoid repeated null-safe calls and improve readability: val response = e.response

Suggested change

	val responseStatus = e.response?.status
	val responseContentType = e.response?.contentType()
	val response = e.response
	val responseStatus = response?.status
	val responseContentType = response?.contentType()

[kpavlov]

Tests are needed.