Implement Distributed Deadline Propagation (Context-Aware Headers)

[cinar]

Implement Distributed Deadline Propagation (Context-Aware Headers)

Overview

This PR implements Distributed Deadline Propagation as specified in issue #44. It adds mechanisms to propagate context deadlines to downstream services and provides early abort functionality for requests with insufficient remaining time.

Implementation Details

• MinDeadlineThreshold: Added configuration to Config and Option to define a minimum threshold for starting an attempt.
• Deadline Middleware: Implemented deadlineMiddleware that checks the remaining context time and returns context.DeadlineExceeded if below the threshold.
• Header Injection: Added InjectDeadlineHeader in propagation.go with support for both standard integer milliseconds and gRPC-compatible timeout formatting.
• Terminal Errors: Updated shouldRetry to treat context.DeadlineExceeded and context.Canceled as terminal errors.
• Example: Added examples/deadline/main.go demonstrating the feature.

Verification

• Added comprehensive unit tests in deadline_propagation_test.go covering early abort, dynamic recalculation, and header injection.
• Verified all tests pass with >90% coverage in the core package.
• Manually verified the behavior using the new example.

fixes #44

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.30435% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.89%. Comparing base (55c53ad) to head (0526183).

Files with missing lines	Patch %	Lines
options.go	60.00%	1 Missing and 1 partial ⚠️
propagation.go	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   91.86%   91.89%   +0.03%     
==========================================
  Files          13       14       +1     
  Lines         897      938      +41     
==========================================
+ Hits          824      862      +38     
- Misses         45       46       +1     
- Partials       28       30       +2     

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