Support Gantt intervals and Y-axis range slider metadata

[mguellsegarra]

Summary

Extend the graph data contract with two XML capabilities:

• y_range="slider" returns vertical range metadata for line and bar graphs while preserving the existing line-axis properties.
• graph type="gantt" returns one start/end interval per record, including record identity and optional grouping labels. Records with the same task name remain separate intervals.

Existing graph definitions keep their current output when neither option is used. The graph processing API remains independent of the client renderer.

Validation

• 142 Mamba examples pass, including legacy graph output, slider metadata, selection and relational labels, duplicate task names, empty results and invalid Gantt definitions.
• Python compilation and git diff --check pass.

Related to gisce/webclient#1110 and gisce/webclient#1240.

Dependencies

This PR has no upstream PR dependency and can merge independently. The ERP and client changes for gisce/webclient#1110 and gisce/webclient#1240 must consume a release containing this contract.

[giscebot]

Reviewed the public graph-processing contract and the related webclient consumers.

• Gantt parsing enforces exactly one X field plus one start and one end field, preserves record identity, and keeps duplicate task names as separate intervals.
• Optional selection/many2one grouping labels and empty/ungrouped results are covered.
• y_range="slider" is opt-in and preserves the existing line yAxisProps payload while adding the renderer-facing metadata for line/bar graphs.
• No dependency, schema, migration, or deployment coupling is introduced in this package.
• Validation: 142 Mamba examples pass on both Python 2.7 and Python 3.11; git diff --check passes.