mcp: cover all branches on nil checks for json response

[nacx]

Description

Fix the a nil check on MCP session to capture all branches.

Related Issues/PRs (if applicable)

N/A

Special notes for reviewers (if applicable)

N/A

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.70%. Comparing base (8766e8c) to head (30eab5e).

Files with missing lines	Patch %	Lines
internal/mcpproxy/handlers.go	0.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2275   +/-   ##
=======================================
  Coverage   84.70%   84.70%           
=======================================
  Files         144      144           
  Lines       21214    21214           
=======================================
  Hits        17970    17970           
  Misses       2162     2162           
  Partials     1082     1082           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.