Add testing and code quality guidelines to CONTRIBUTING.md (#2685)

Add explicit requirements for testing (E2E and unit tests) and code quality
expectations including PR size limits, code review responsibilities, and
manual testing requirements.

Signed-off-by: Juan Antonio Osorio <[email protected]>

Author: JAORMX

CONTRIBUTING.md

@@ -69,6 +69,25 @@ are a great place to start!
 - Once approved, all of your commits will be squashed into a single commit with
   your PR title.
 
+### Testing requirements
+
+- Add end-to-end tests for new features covering both API and CLI flows.
+- Write unit tests for new code alongside the source files.
+
+### Code quality expectations
+
+Pull request authors are responsible for:
+
+- Keeping PRs small and focused. PRs exceeding 1000 lines may be blocked and
+  require splitting into multiple PRs or logical commits before review. If a
+  large PR is unavoidable, include an explanation in the PR description
+  justifying the size and describing how the changes are organized for review.
+- Reviewing all submitted code, regardless of whether it's AI-generated or
+  hand-written.
+- Manually testing changes to verify new or existing features work correctly.
+- Ensuring coding style guidelines are followed.
+- Respecting architecture boundaries and design patterns.
+
 ### Contributing to docs
 
 The ToolHive user documentation website is maintained in the