Respect STAC cube:dimensions as source of truth in metadata_from_stac + align tests with dimension policy

[suriyahgit]

Problem

metadata_from_stac() previously injected openEO default dimensions (x, y, t, bands) even when a STAC object already defined dimensions via cube:dimensions.

This led to:

• dimension names being overridden or altered,
• inconsistencies between STAC metadata and openEO metadata,
• tests relying on implicit default behavior instead of STAC semantics.

Fix

Introduce a clear dimension resolution policy:

• If cube:dimensions is present
  → treat it as the source of truth and preserve dimension names and order.

• If cube:dimensions is absent
  → fall back to openEO defaults (x, y, optional t).

The metadata parsing logic was refactored accordingly and aligned with PySTAC datacube capabilities.

Testing

• Updated existing tests to no longer assume an always-present band dimension.

• Added checks verifying:

  • fallback behavior when cube:dimensions is missing,
  • preservation of dimension names when cube:dimensions exists,
  • correct temporal dimension handling across STAC object types.

• Tests now explicitly enforce the new dimension policy to prevent regressions.

This change makes metadata parsing more STAC-compliant while keeping backward-compatible fallback behavior.

Refer:
https://github.com/destine-datalake-cube/dedl-openeo-coordination/issues/3
Also solves: #743