fix: improve compatibility with non-compliant MCP servers (#413)

Addresses issues with servers like Shopify that violate MCP/HTTP specs:
- Prioritize application/json in Accept headers to fix content-type issues
- Handle non-compliant notification non-empty responses
- Add status code validation and null safety improvements

Resolves #406

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>