feat: replace the internal plugin framework with CPEX

[araujof]

Summary

Replace the internal plugin framework (mcpgateway/plugins/framework/, mcpgateway/plugins/tools/, plugin_templates/) with the CPEX external package (cpex>=0.1.0.dev10).

Closes: #3753

What changed

• Added cpex as external dependency in pyproject.toml
• Migrated all imports: from mcpgateway.plugins.framework → from cpex.framework, from mcpgateway.plugins.tools → from cpex.tools
• Moved get_plugin_manager() singleton factory to mcpgateway/plugins/__init__.py (bridges cpex with gateway-specific hook policies)
• Migrated PluginMode enums: ENFORCE → SEQUENTIAL, PERMISSIVE → TRANSFORM, ENFORCE_IGNORE_ERROR → SEQUENTIAL + on_error: ignore
• Adapted to cpex API changes:
  • PluginViolation.http_status_code / .http_headers → violation.details["http_status_code"] / details["http_headers"]
  • PromptPosthookPayload.name → .prompt_id; ToolPreInvokePayload.arguments → .args
• Deleted internal framework: mcpgateway/plugins/framework/, mcpgateway/plugins/tools/, plugin_templates/ directory, tests/unit/mcpgateway/plugins/framework/
• Created acceptance tests (tests/acceptance/plugins/test_cpex_contract.py) verifying cpex API surface, behavioral contracts, serialization, and settings compatibility
• Updated YAML configs across plugins and test fixtures
• Updated documentation under docs/docs/using/plugins/, plugins/README.md, plugins/AGENTS.md, llms/plugins-llms.md

Test results

• Unit tests pass
• Acceptance tests pass
• Performance profiling shows no regression
• flake8, bandit, pylint all clean