state: support mustache form for placeholders

[WineChord]

No description provided.

[WineChord]

agno 用的是 {{}} 形式，我们提供的是 {} 的形式，做了一下兼容。

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.69757%. Comparing base (cf0896e) to head (479690e).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/state/state_injection.go	57.14286%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #517         +/-   ##
===================================================
+ Coverage   70.68538%   70.69757%   +0.01219%     
===================================================
  Files            195         195                 
  Lines          29852       29861          +9     
===================================================
+ Hits           21101       21111         +10     
+ Misses          7455        7454          -1     
  Partials        1296        1296                 

Flag	Coverage Δ
unittests	70.69757% <57.14286%> (?)

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[sandyskies]

LGTM