Refactor chat to chat service and chat plugin

[ananzh]

Description

Issues Resolved

Screenshot

Testing the changes

Changelog

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

❌ Patch coverage is 56.37255% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.75%. Comparing base (a47de88) to head (f8201f7).

Files with missing lines	Patch %	Lines
.../chat/server/routes/ml_routes/generic_ml_router.ts	5.55%	34 Missing ⚠️
src/plugins/chat/public/plugin.ts	20.68%	19 Missing and 4 partials ⚠️
src/plugins/chat/public/services/chat_service.ts	54.16%	10 Missing and 1 partial ⚠️
src/core/public/chat/chat_service.ts	84.84%	5 Missing ⚠️
...ns/chat/server/routes/ml_routes/router_registry.ts	44.44%	4 Missing and 1 partial ⚠️
src/plugins/chat/server/routes/index.ts	82.60%	2 Missing and 2 partials ⚠️
...ns/chat/server/routes/ml_routes/ml_agent_router.ts	33.33%	2 Missing ⚠️
src/core/public/chat/no_op_chat_service.ts	92.85%	1 Missing ⚠️
src/plugins/chat/public/components/chat_window.tsx	80.00%	1 Missing ⚠️
src/plugins/chat/public/services/ag_ui_agent.ts	50.00%	0 Missing and 1 partial ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10983      +/-   ##
==========================================
- Coverage   60.79%   60.75%   -0.04%     
==========================================
  Files        4531     4537       +6     
  Lines      122262   122401     +139     
  Branches    20501    20535      +34     
==========================================
+ Hits        74325    74361      +36     
- Misses      42695    42787      +92     
- Partials     5242     5253      +11     

Flag	Coverage Δ
Linux_1	?
Linux_2	?
Linux_3	?
Linux_4	?
Windows_1	26.55% <5.55%> (-0.02%)	⬇️
Windows_2	38.95% <88.88%> (+0.05%)	⬆️
Windows_3	39.47% <33.33%> (-0.05%)	⬇️
Windows_4	33.71% <11.66%> (-0.03%)	⬇️

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[wanglam]

It seems dataSource isn't being used in the explore plugin. Can we remove this dependency?