fix: validate include 'taskfile' field, suggest 'vars:' if missing

[RichardHopperProGrammar]

Problem

When a user accidentally writes includes: with variable-like content instead of vars: — a common typo like this:

# Taskfile.yml
version: '3'

includes:
  GOBIN:
    sh: echo $(go env GOPATH)/bin

tasks:
  test:
    cmds:
      - "{{.GOBIN}}/richgo test ./..."

...the current behavior is to silently accept the malformed include entry (since any YAML mapping passes the yaml.MappingNode check in Includes.UnmarshalYAML), and later report a misleading "include cycle detected between A <--> A" error, because the empty Taskfile path resolves to the same file.

This is confusing and unhelpful — the real problem isn't a cycle, it's that the user put variables under includes: instead of vars:.

Solution

Added validation in taskfile/ast/include.go to check that each include entry has a non-empty Taskfile field after decoding. If it doesn't:

1. Returns a clear error: include "GOBIN" is missing the required "taskfile" field; did you mean to use "vars:" instead?
2. Lists unexpected keys found (e.g., sh) so the user understands what went wrong
3. Provides actionable guidance (use vars: instead)

Changes

• taskfile/ast/include.go: Added validation in Includes.UnmarshalYAML after decoding each include value, checking for empty Taskfile and producing a helpful error message
• testdata/includes_missing_taskfile/: Test fixtures reproducing the exact scenario from issue Incorrect/confusing include cycle detected error message after a silly mistake #1881
• task_test.go: New test TestIncludesMissingTaskfile verifying the new error message is shown instead of the misleading cycle error

Testing

• All existing include-related tests pass (TestIncludes, TestIncludeCycle, TestIncludesIncorrect, etc.)
• New test specifically validates:
  • Error contains "missing the required "taskfile" field"
  • Error does NOT contain "include cycle detected"

[trulede]

See #1902

[RichardHopperProGrammar]

Thanks @trulede — great call. #1902 already covers this and has been through review, so I'll close this PR to avoid duplication. Good to know about the existing work!

[trulede]

@RichardHopperProGrammar I think the other PR is dormant .... so you might prefer to keep yours open, and incorporate the feedback. Small concise changes that don't change too much have a good chance of being accepted.

[RichardHopperProGrammar]

Thanks @trulede — I'll keep it open then. Good call about #1902 being dormant and the project moving away from go generate. Let me incorporate the feedback (switch to fmt.Errorf with %w, add test for the include cycle error) and push an update soon. Appreciate the guidance!

[RichardHopperProGrammar]

Thanks @trulede — I see #1902 is dormant so I will keep mine open and incorporate feedback. The test for missing taskfile vs. cycle error is already included (line 1069 in task_test.go — verifies the error says "taskfile not found" and not "include cycle detected"). Happy to make any other refinements you suggest.