chore(context): test context initialization and handler logic

[appleboy]

if it needs to check if the handler is nil, tie c.index shall always ++.

add testing case for #4083 credit by @zjj

• Test with no handlers: Verifies that calling Next() on a context with no handlers does not change the index property.
• Test with one handler: Checks that a single handler is executed, the index is updated correctly, and the handler sets a key-value pair in the context.
• Test with multiple handlers: Ensures that multiple handlers are executed in sequence, the index is updated correctly, and each handler can set key-value pairs in the context.
• Key existence and value check: Confirms that the keys set by the handlers exist in the context and have the expected values.
• Handler chaining: Validates that handlers can call Next() to pass control to the next handler in the chain and still perform actions before and after the call to Next().

cc @zjj

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.99%. Comparing base (3dc1cd6) to head (151f477).
Report is 80 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4087      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     3387     +205     
==========================================
+ Hits         3157     3353     +196     
- Misses         17       24       +7     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
-tags "sonic avx"	98.98% <100.00%> (?)
-tags go_json	98.98% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	98.99% <100.00%> (-0.22%)	⬇️
go-1.22	98.99% <100.00%> (?)
macos-latest	98.99% <100.00%> (-0.22%)	⬇️
ubuntu-latest	98.99% <100.00%> (-0.22%)	⬇️

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[appleboy]

@thinkerou Please help to review.