Add helper macros to Rust

[logan-gatlin]

Add kj::KjError construction macros

Adds Rust macros for building cxx::KjError values, mirroring C++'s
KJ_EXCEPTION/KJ_FAIL_REQUIRE/KJ_UNIMPLEMENTED macros (kj/debug.h). One
pair per kj::Exception::Type variant: a construct-only macro that evaluates
to a KjError value, and an early-return macro that does return Err(...)
from the enclosing function.

KJ exception type	Construct-only	Early-return
Failed	kj::failed!	kj::fail_require!
Overloaded	kj::overloaded!	kj::overloaded_require!
Disconnected	kj::disconnected!	kj::disconnected_require!
Unimplemented	kj::not_implemented!	kj::not_implemented_require!

The Unimplemented pair is named not_implemented! rather than
unimplemented! to avoid shadowing std::unimplemented! at any use kj::unimplemented; import site.

[ask-bonk]

APIError: Invalid Anthropic API Key

github run

[ask-bonk]

APIError: Invalid Anthropic API Key

github run

[ask-bonk]

APIError: Invalid Anthropic API Key

github run

[ask-bonk]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.79%. Comparing base (78f2814) to head (6efde75).
⚠️ Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7014      +/-   ##
==========================================
- Coverage   68.49%   67.79%   -0.70%     
==========================================
  Files         438      468      +30     
  Lines      130184   132593    +2409     
  Branches    21422    21515      +93     
==========================================
+ Hits        89167    89897     +730     
- Misses      27977    29594    +1617     
- Partials    13040    13102      +62     

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