tpl/collections: Include template path in IsSet unsupported type warning

[truffle-dev]

Fixes #11794

Summary

When isset is called on a value that is not an array, channel, slice, or map, Hugo logs:

WARN  calling IsSet with unsupported type "string" (string) will always return false.

On any site with more than one template there's no way to find the offending call site. The warning is also emitted via Log.Warnf, so it has no ID and can't be silenced via ignoreLogs.

This PR threads context.Context through IsSet, looks up the executing template via tpl.Context.CurrentTemplate.Get, and surfaces the filename in the warning. It also switches Log.Warnf to Log.Warnidf with a new constant WarnIsSetUnsupportedType so the warning is groupable and suppressible.

After the change:

WARN  calling IsSet with unsupported type "string" (string) in "/layouts/home.html" will always return false.
You can suppress this message by adding the following to your site configuration:
ignoreLogs = ['warning-isset-unsupported-type']

Changes

• tpl/collections/collections.go — IsSet accepts ctx context.Context as its first arg, matching the pattern used by Delimit and other ctx-aware namespace functions. The default branch resolves the current template and appends in "<path>" to the warning, then routes through Log.Warnidf.
• common/constants/constants.go — adds WarnIsSetUnsupportedType.
• tpl/collections/collections_test.go — passes context.Background() to the existing TestIsSet cases.
• tpl/collections/collections_integration_test.go — adds a regression test asserting the template path appears in the warning.
• docs/data/docs.yaml — adds ctx to IsSet's Args list, matching the documented shape for other ctx-aware namespace functions.

Tests

• go test ./tpl/collections/... -run TestIsSet -v — unit and integration tests pass.
• go test ./tpl/collections/... — full package green.
• go vet ./tpl/collections/... — clean.
• gofmt -l on touched paths — clean.
• go build ./... — full tree builds.

AI Assistance

I used AI assistance to investigate the issue, design the fix, and prepare tests. I reviewed the final diff, kept the scope narrow per CONTRIBUTING.md, and ran the checks listed above before opening.

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[truffle-dev]

Hi @bep — small focused fix against #11794 (open since 2023), ~13 days idle. Happy to rebase or adjust the warning message if anything looks off. No rush, just keeping it on your radar.