feat: implement default data converters

[ugur-vaadin]

Description

Adds DefaultDataConverter that supports 15 chart type patterns via column-name-based detection, plus a fallback for basic charts. DefaultDataConverter supports

• 15 chart type patterns via column-name-based detection
• a fallback for basic charts
• multi-series grouping via a series column

The patterns checked:

• OHLC (open, high, low, close)
• BoxPlot (low, q1, median, q3, high)
• Organization (id, name, parent, title)
• Gantt (name, start, end + optional id, parent, dependency, completed)
• Treemap (id, parent, value)
• Sankey (from, to, weight)
• Heatmap (x, y, value)
• XRange (x, x2, y)
• Timeline (name, label, description)
• Bubble (x, y, z)
• Flags (title, optionally text)
• Range (low, high)
• Bullet (y, target)
• Waterfall (y, waterfall_type)
• Fallback (first category + first numeric column)

Fixes https://github.com/orgs/vaadin/projects/103/views/1?filterQuery=&pane=issue&itemId=165657712

Type of change

• Bugfix
• Feature

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.
• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[sonarqubecloud]

Quality Gate passed

Issues
11 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

[tomivirkki]

"series" is a pretty common column name — e.g. a table tracking TV shows could easily have a series column for the show name. Consider using something more specific to avoid accidental collisions with real data columns.

[tomivirkki]

Is it necessary to have all these options for the x-column name? I'd assume we can make the system prompt use a consistent column name for x.