fix: correct first element condition in ImportTemplate loops (fixes #358)

[Cypher-CP0]

Linked issue

• Closes [Bug] ImportTemplate uses incorrect condition (obj == 0), preventing assignment of stable base IDs #358
• Related #

Existing related work reviewed

• Issues/PRs reviewed: [Bug] ImportTemplate uses incorrect condition (obj == 0), preventing assignment of stable base IDs #358
• If none found, write: None found after search

Overlap assessment

• Classification: Bug fix
• Overlapping items: None
• Why this is not duplicate/superseded: No existing PR addresses this bug

Why this PR should proceed

• Silent logic error causing all imported entities to receive random IDs
  instead of stable base IDs, breaking deterministic model initialization
  and any downstream logic depending on TEC_0, COM_0, EMI_0, STG_0

Summary

• What changed: Added enumerate() to the four affected loops
  (technologies, commodities, emissions, storages) in ImportTemplate.py
  and replaced obj == 0 with i == 0 in each. Also fixed an incorrect
  default description string "Default commodity" inside the technologies
  loop (line 633).
• Why: obj is a dictionary, not an integer, so obj == 0 always
  evaluates to False. This meant first=True was never passed,
  so stable base IDs (TEC_0, COM_0, EMI_0, STG_0) were never assigned
  to the first element of each entity type during template import.

Validation

• Tests added/updated (or not applicable)
• Validation steps documented
• Evidence attached (logs/screenshots/output as relevant)

Documentation

• Docs updated in this PR (or not applicable)
• Any setup/workflow changes reflected in repo docs

Scope check

• No unrelated refactors
• Implemented from a feature branch
• Change is deliverable without upstream OSeMOSYS/MUIO dependency
• Base repo/branch is EAPD-DRB/MUIOGO:main (not upstream)

Exception rationale