fix: define inflow_t before branch so it is always assigned

[daniel-rdt]

Changes proposed in this Pull Request

This PR introduces a minor fix ensuring inflow_t is always defined in attach_hydro, resolving a pylint use-before-assignment issue.

Checklist

Required:

• Changes are tested locally and behave as expected.
• Code and workflow changes are documented.
• A release note entry is added to doc/release_notes.md.
• The description is human-written and any AI-generated content is marked.

If applicable:

• Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• New rules are documented in the appropriate doc/*.md files.