feat: Rest API endpoints from roadmap

[sarthakNITT]

Description

• Implements true pause/resume HTTP endpoints for the session execution API.
• Previously, POST /api/sessions/{id}/pause was wired to handle_stop, making pause and stop identical.
• This PR separates them into distinct behaviours with correct state management and resume guards.
• Updated handle_resume() to check for status. If paused then run from the paused node, if cancelled then return with error telling to re start using trigger endpoint
• handle_pause() now pause the currently running and cron future running processes and stores their id and node

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #5538

Changes Made

• execution_stream.py - Added _pause_requested: set[str] to track pause vs cancel intent. Added pause_execution() method that flags an execution as paused before cancelling its task. Updated the CancelledError handler to write status="paused" to disk when the flag is set, vs status="cancelled" for hard stops.
• routes_execution.py - Added handle_pause() handler: calls stream.pause_execution() and runtime.pause_timers() to freeze both the live execution and any scheduled (cron) triggers. Updated handle_resume() to call resume_timers() after re-triggering, and added a 409 guard that rejects resume attempts on status="cancelled" sessions with a clear error. Rewired POST /pause route from handle_stop → handle_pause.
• test_api.py - Added pause_execution() to MockStream, pause_timers()/resume_timers() to MockRuntime. Added test_pause_success (asserts {"paused": true} and pause_timers() called), test_resume_rejected_for_cancelled_session (asserts 409), and updated test_resume_from_session_state to assert resume_timers() is called.
• docs/ — Updated core/framework/server/README.md (description and example for /pause), docs/server-cli-arch.md (corrected /stop label, added /pause row), docs/roadmap.md (marked pause, resume, stop, start, status endpoints as [x]).

Testing

Describe the tests you ran to verify your changes:

• Unit tests pass (cd core && pytest tests/)
• Lint passes (cd core && ruff check .)
• Manual testing performed

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

Add screenshots to demonstrate UI changes.