fix(web): only open slash variable picker when / is typed (#39755)

[Souravrajvi0]

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Summary

• Open the workflow slash variable picker only when the user inserts / via text input
• Ignore caret placement on existing / characters in prompt content (for example URLs)
• Track slash intent with beforeinput and clear it on pointer clicks so re-clicking an old slash does not reopen the menu
• Add regression coverage for typed / vs clicking after an existing slash in URL text

Fixes #39755

Root cause

ComponentPickerBlock used useBasicTypeaheadTriggerMatch with minLength: 0 for the / trigger. That matches any caret position immediately after an existing slash, so clicking inside URL text like https://... reopened the variable picker even though the user did not type /.

Impact

Workflow Agent prompt editing no longer opens the variable picker when users click inside existing prompt text that happens to contain /. Typing / still opens the picker as before, including when filtering with additional characters after the slash.

Screenshots

Before	After
N/A — interaction fix verified with unit tests	N/A — interaction fix verified with unit tests

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran targeted frontend unit tests for the changed prompt-editor picker path.

Validation

cd web && pnpm exec vp test run app/components/base/prompt-editor/plugins/component-picker-block/__tests__/index.spec.tsx

• 20 passed (including regression test for typing / vs clicking after an existing slash)