fix(tracker-client): improve error message when JSON config is not well-formatted (#1042)

[josecelano]

Summary

Fixes #1042 — Tracker Checker (HTTP): Improve Error Message When JSON Config Is Not Well-Formatted

When the Tracker Checker received a malformed JSON configuration (e.g. a trailing comma), it
panicked with a generic invalid config format message. The detailed parse error was buried in
the "Caused by" chain, invisible without RUST_BACKTRACE=1.

Changes

• console/tracker-client/src/console/clients/checker/error.rs (new): ConfigSource enum
  (env var or file path) and AppError enum (InvalidConfig / Runtime) with JSON serialization
  to the Tracker CLI I/O Contract schema.
• console/tracker-client/src/console/clients/checker/app.rs: Refactored setup_config and
  load_config_from_file to return AppError instead of anyhow. Parse errors now propagate
  with source context instead of a generic wrapper.
• console/tracker-client/src/bin/tracker_checker.rs: Replaced .expect() panic with
  structured error handling — JSON error written to stderr, exits with code 2 for config errors.
• console/tracker-client/src/console/clients/checker/config.rs: Added 5 new unit tests for
  parse error propagation.
• console/tracker-client/tests/tracker_checker.rs (new): 9 end-to-end integration tests
  covering env var and file config sources.

Error Output (before vs. after)

Before:

thread 'main' panicked at console/tracker-client/src/bin/tracker_checker.rs:6:22:
Some checks fail: invalid config format

Caused by:
    JSON parse error: trailing comma at line 7 column 5

After:

{"error":{"kind":"invalid_configuration","source":"TORRUST_CHECKER_CONFIG","message":"JSON parse error: trailing comma at line 7 column 5"}}

Exit code: 2

Testing

• 35 unit tests pass (including 12 new)
• 9 integration tests pass (new)
• All linters pass (linter all exits 0)
• Manual verification with tracker-demo URLs documented in the issue spec

Issue Spec

docs/issues/open/1042-tracker-checker-http-improve-error-message-json-config.md

[Copilot]

Pull request overview

This PR updates the tracker_checker CLI to surface configuration/JSON parsing failures as contract-compliant structured JSON errors (stderr) with deterministic exit codes, instead of panicking with a generic message.

Changes:

• Introduces AppError/ConfigSource for checker-specific error classification and formatting.
• Refactors config loading/parsing to return structured config errors and maps runtime failures to exit code 1.
• Adds unit + integration tests for malformed/missing configuration and exit code semantics.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
docs/issues/open/1042-tracker-checker-http-improve-error-message-json-config.md	Updates the issue spec to reference the CLI I/O contract and acceptance evidence.
console/tracker-client/tests/tracker_checker.rs	Adds E2E tests asserting stderr JSON envelope + exit code 2 for config errors.
console/tracker-client/src/console/clients/checker/mod.rs	Exposes the new error module.
console/tracker-client/src/console/clients/checker/error.rs	Adds structured AppError + config source tracking and JSON error envelope generation.
console/tracker-client/src/console/clients/checker/config.rs	Adds unit tests ensuring parse/validation error details are preserved.
console/tracker-client/src/console/clients/checker/app.rs	Replaces anyhow config errors with AppError and propagates parse details.
console/tracker-client/src/bin/tracker_checker.rs	Replaces panic-on-error with JSON-to-stderr + exit code handling.
console/tracker-client/Cargo.toml	Adds tempfile dev-dependency for integration tests.
Cargo.lock	Locks tempfile dependency.

---

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[codecov]

Codecov Report

❌ Patch coverage is 94.93671% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.05%. Comparing base (5ca4de6) to head (36ed8ef).

Files with missing lines	Patch %	Lines
...racker-client/src/console/clients/checker/error.rs	93.54%	6 Missing ⚠️
console/tracker-client/src/bin/tracker_checker.rs	80.00%	0 Missing and 1 partial ⚠️
.../tracker-client/src/console/clients/checker/app.rs	95.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1764      +/-   ##
===========================================
- Coverage    79.44%   79.05%   -0.40%     
===========================================
  Files          369      370       +1     
  Lines        27328    27682     +354     
  Branches     27328    27682     +354     
===========================================
+ Hits         21710    21883     +173     
- Misses        5337     5517     +180     
- Partials       281      282       +1     

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[josecelano]

ACK 36ed8ef