refactor: improve instructions for ambiguous values

[ugur-vaadin]

Description

There is an issue with FormAIController ignoring per-field description directives like "let the user pick when the query returns multiple items" on queryable fields. When the query returned multiple matches, the LLM picked one (or all) of them rather than asking the user, regardless of what the developer wrote in the description.

This PR updates the form-fill workflow text in INSTRUCTIONS_TEXT step 3 so the LLM checks the field's description for an ambiguity directive before writing a queryable field whose query returned multiple matches. When the directive is present and the user's prompt does not single out the subset to write, the LLM asks for clarification instead. Prompts like "Add both X and Y" or "Pick all of them" already name the subset, so they proceed without asking. Without a directive, the LLM picks the most reasonable match and writes it.

Based on Form Filler DX test session findings.

No related issue.

Type of change

• Bugfix
• Feature
• Refactor

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/overview
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud