chore: replace serdeconv with toml and serde_json

[zwpaper]

Summary

This change replaces serdeconv with toml and serde_json crates directly.
This is to add support for multiple lines table in toml for our config.

Test plan

Manually verified by Wei:

• llama read model.json
• event service to json in log
• read config
• language from toml
• model registry to/from json

🤖 Generated with Pochi | Task

[codecov]

Codecov Report

❌ Patch coverage is 23.52941% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.46%. Comparing base (e19008a) to head (311a1b9).
⚠️ Report is 3 commits behind head on next.

Files with missing lines	Patch %	Lines
crates/tabby-common/src/config.rs	28.57%	5 Missing ⚠️
crates/tabby-common/src/registry.rs	33.33%	4 Missing ⚠️
crates/llama-cpp-server/src/lib.rs	0.00%	3 Missing ⚠️
crates/tabby/src/services/event.rs	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (23.52%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4435      +/-   ##
==========================================
+ Coverage   54.35%   54.46%   +0.11%     
==========================================
  Files         227      227              
  Lines       27121    26935     -186     
==========================================
- Hits        14741    14671      -70     
+ Misses      12380    12264     -116     

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