Fix context leak in rule/link check loop

[Nepomuk5665]

Summary

• Fix resource leak caused by defer cancel() inside a for loop
• The cancel functions would accumulate during the loop and only be called when the function returns
• Extract HTTP request logic to a separate checkLink() method where defer properly releases resources

Bug Description

In RuleLinkCheck.Check(), a context with timeout was created inside a for loop for each annotation being checked:

for _, ann := range entry.Rule.AlertingRule.Annotations.Items {
    // ...
    rctx, cancel := context.WithTimeout(ctx, c.timeout)
    defer cancel()  // BUG: only called when Check() returns, not at end of iteration
    // ...
}

When checking multiple link annotations, the cancel functions would stack up and only be called when Check() returns, not at the end of each loop iteration. This causes:

1. Context resources to leak during the loop
2. Potential goroutine leaks from uncanceled contexts
3. Memory pressure from accumulated cancel functions

Fix

Extract the HTTP request logic into a separate checkLink() method. The defer cancel() in this new method correctly releases resources when each individual link check completes.

Testing

All existing tests pass:

=== RUN   TestRuleLinkCheck
--- PASS: TestRuleLinkCheck (0.01s)