Fix Slack update_message global URL validation

[fallintoplace]

Fixes #5164.

This keeps Slack update_message validation from dereferencing a missing local api_url during YAML unmarshalling, then validates again after global Slack defaults have been applied. That lets a global chat.postMessage URL work while still rejecting webhook URLs with the existing readable error.

Validation:

• go test ./config -run 'TestSlackUpdateMessage|TestSlackGlobalAppToken|TestSlackNoAPIURL|TestSlackGlobalAPIURLFile'
• go test ./config ./notify/slack
• go test ./config -run TestSlackUpdateMessageWithGlobalAPIURL -count=1
• go test ./... does not complete in this checkout because generated UI assets and the built alertmanager/amtool acceptance-test binaries are missing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Enterprise

Run ID: 01f11073-f0d6-4ebd-97cd-17a1f4931251

📥 Commits

Reviewing files that changed from the base of the PR and between 364a227 and 2390ce7.

📒 Files selected for processing (3)

• config/config.go
• config/config_test.go
• config/notifiers.go

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📝 Walkthrough

Walkthrough

This PR fixes a nil pointer panic when update_message: true is used in Slack configs without an explicit api_url but with a global slack_api_url. It adds validation to enforce that update_message requires a valid chat.postMessage API URL and integrates this check into config unmarshalling.

Changes

Slack Update Message Validation

Layer / File(s)	Summary
Slack update message validation contract and function config/notifiers.go	Defines slackUpdateMessageAPIURL and slackUpdateMessageError constants for the required chat.postMessage endpoint, and implements validateSlackUpdateMessageURL() to enforce URL validation when UpdateMessage is enabled and APIURL is set.
Config unmarshalling validation integration config/config.go	Integrates Slack validation into Config.UnmarshalYAML by calling validateSlackUpdateMessageURL() after receiver processing to propagate validation errors during config loading.
Test coverage for global Slack API URL scenarios config/config_test.go	Updates TestSlackUpdateMessageWebhookURL to use the validation error constant, adds TestSlackUpdateMessageWithGlobalAPIURL to verify global slack_api_url enables update_message, and adds TestSlackUpdateMessageWithGlobalWebhookURL to verify webhook-style URLs trigger validation errors.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: fixing Slack update_message validation to work with global URL configuration.
Description check	✅ Passed	The description addresses the linked issue, explains the problem and solution, and provides validation commands, though the release-notes section is empty.
Linked Issues check	✅ Passed	The PR successfully addresses issue #5164 by fixing the nil pointer dereference panic and validating update_message after global defaults are applied.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the Slack update_message validation issue; no out-of-scope modifications detected.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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