Fix Slack API error handling in notify hook

[shunichironomura]

Warning

This content was written by an AI agent and must be verified by a human developer. After human verification, this alert may be removed.

Summary

• validate the JSON-level ok field returned by chat.postMessage
• surface Slack's error value instead of recording failed notifications as successful
• preserve timestamp extraction for successful responses and add regression coverage

Fixes #1079.

AI assistance

• AI used: yes — Amp investigated the issue, implemented the fix, and added tests
• Human review of AI-assisted code: pending

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied

Verification

• cargo fmt --check --all
• cargo test -p capsula-notify-slack
• cargo clippy -p capsula-notify-slack --all-targets --all-features -- -D warnings

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.49%. Comparing base (433b438) to head (673284a).

Files with missing lines	Patch %	Lines
crates/capsula-notify-slack/src/lib.rs	93.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1224      +/-   ##
==========================================
+ Coverage   54.16%   54.49%   +0.32%     
==========================================
  Files          44       44              
  Lines        4233     4259      +26     
==========================================
+ Hits         2293     2321      +28     
+ Misses       1940     1938       -2     

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