Fix 1415: observability_exclude_paths env parsing issue

[nmveeresh]

Closes: #1415

Fix observability_exclude_paths Env Parsing Issue

Summary:
This PR addresses the SettingsError encountered when loading the OBSERVABILITY_EXCLUDE_PATHS environment variable in mcpgateway. The error occurred because the environment variable was previously provided as a comma-separated string, which pydantic-settings could not parse into a List[str].

Changes:

• Updated config.py to ensure observability_exclude_paths is correctly parsed from environment variables as a JSON array.

• Modified .env example and documentation to use JSON array format:

  OBSERVABILITY_EXCLUDE_PATHS='["/health", "/healthz", "/ready", "/metrics", "/static/.*"]'
  

Impact:

• Prevents runtime errors on application startup due to settings parsing issues.
• Compatible with .env files, shell exports, and Helm/Kubernetes environment variables.
• Ensures observability paths are correctly loaded and applied for tracing exclusion.

Testing:

• Verified settings.observability_exclude_paths loads correctly from all sources.
• Confirmed that default values are applied when the env variable is unset.
• Local and containerized setups tested to ensure no regressions.

  python -c "from mcpgateway.config import settings; print(settings.observability_exclude_paths)"

Output:

['/health', '/healthz', '/ready', '/metrics', '/static/.*']