fix: improve error messages for invalid template tags in HTML start tags

[dupontbertrand]

Summary

• The pattern <input {{#if isDisabled}}disabled{{/if}}> causes a silent compiler crash with a misleading error: Cannot find module '../.././imports/ui/.../template.html' and Template.xxx is not defined. Developers waste hours debugging a module resolution issue that is actually a Spacebars syntax problem.
• This PR replaces the cryptic failure with specific, actionable error messages for each invalid pattern:
  • {{#if}}/{{#unless}}/{{#each}}/{{#with}} in start tag: suggests the attribute="{{helper}}" pattern with Bad/Good syntax example
  • {{{triple-stache}}} in start tag: explains the raw HTML issue
  • {{> inclusion}} in start tag: clear rejection message
  • Custom block helpers: specific message mentioning dictionary helpers
• Add test coverage with string assertions for each error path

Before

Cannot find module '../.././imports/ui/pages/myPage/template.html'
Template.myTemplate is not defined

After

{{#if}} is not allowed in an HTML start tag. To conditionally add a boolean
attribute like "disabled", "checked", or "selected", use the
attribute="{{helper}}" syntax instead.
  Bad syntax:  <input {{#if isDisabled}}disabled{{/if}}>
  Good syntax: <input disabled="{{isDisabled}}">

Test plan

• Tested manually with a Meteor 3.4 app using local package symlinks
• Verified {{#if}} standalone attribute shows new message with Bad/Good example
• Verified {{{triple-stache}}} in start tag shows raw HTML message
• Verified {{> inclusion}} in start tag shows inclusion message
• Verified valid case ({{#if}} inside attribute value) still compiles and runs
• Tinytest suite (spacebars-compiler - parse) passes — 416/416 tests, 0 failures

[jankapunkt]

@dupontbertrand @wreiske can you please check if this may conflict with pr #481

[wreiske]

I believe #481 does cover this use case, yes.

[dupontbertrand]

My bad, don't know something like that was already there ! @wreiske @jankapunkt

[wreiske]

No worries at all! It shows that multiple people thought it needed improvement so there's some validation there! ☺️