fix(generic): resolve ConfigFromWebhookURL regression causing 400 errors

[nicholas-fedor]

This PR fixes a regression in the Generic service's ConfigFromWebhookURL function that was causing 400 Bad Request errors when using webhook URLs with configuration parameters (like ?template=json).

Problem

The issue was reported in nicholas-fedor/watchtower#826 where 400 errors were experienced with Notifiarr notifications using the Generic service, while Discord notifications worked fine. The root cause was that config properties from webhook URL query parameters were not being set, causing incorrect defaults (plain text instead of JSON payloads).

Solution

• Fixed ConfigFromWebhookURL: Simplified the function to properly set config properties from webhook URL query parameters by calling SetConfigPropsFromQuery directly on unescaped query parameters after stripping custom values.
• Enhanced testing: Added regression tests and testable examples for ConfigFromWebhookURL functionality.

Additional Changes

• Improved code quality: Added detailed inline comments.
• Documentation updates: Created godoc with testable examples and updated MkDocs documentation.

Changes

• pkg/services/specialized/generic/generic_config.go: Fixed ConfigFromWebhookURL regression
• pkg/services/specialized/generic/generic.go: Added inline comments
• pkg/services/specialized/generic/custom_query.go: Added inline comments
• pkg/services/specialized/generic/generic_test.go: Integrated regression tests
• pkg/services/specialized/generic/doc.go: Added comprehensive godoc documentation
• pkg/services/specialized/generic/generic_example_test.go: Added testable examples
• docs/services/specialized/generic/index.md: Updated MkDocs documentation
• docs/services/specialized/generic/config.md: Created configuration reference

[codecov]

Codecov Report

❌ Patch coverage is 95.77465% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/services/specialized/generic/generic.go	92.30%	2 Missing ⚠️
pkg/services/specialized/generic/custom_query.go	91.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ
pkg/services/specialized/generic/generic_config.go	93.61% <100.00%> (+8.32%)	⬆️
pkg/services/specialized/generic/custom_query.go	91.11% <91.66%> (+1.36%)	⬆️
pkg/services/specialized/generic/generic.go	81.88% <92.30%> (+2.27%)	⬆️

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