Partial include filter marks section/hot-water group as fully validated

[liudger]

Found during review of #1519 (behavior-preserving refactor — this is pre-existing behavior moved verbatim into SectionValidator, not a regression introduced by that PR).

Problem

Lazy validation uses a single boolean "done" predicate that ignores which parameters were actually validated:

• ensure_section_validated() → APIValidator.is_section_validated(section)
• ensure_hot_water_group_validated() → group_name in self._validated_hot_water_groups

Because validate_section() marks the entire section/group as validated even when called with include=[subset], a first call with an include filter makes later calls (a different include, or include=None) skip validation entirely. Consequences:

1. The hot-water cache can stay permanently incomplete for that group.
2. Skipped sections can later request unsupported params; combined with zip(..., strict=True) in _request_named_params(), missing keys in the device response can raise ValueError at fetch time.

Affected code (src/bsblan/_validation.py)

• ensure_section_validated() — is_section_validated(section) used as the sole "done" predicate.
• ensure_hot_water_group_validated() — when an include filter removes all of a group's params, the empty group is still added to _validated_hot_water_groups.
• ensure_hot_water_group_validated() — group marked validated after only a subset of params were checked/cached.

Possible directions

• Track the validated parameter set per section/group, or
• Do not persist the "validated" flag when an include filter was applied, or
• Re-validate when requested params are absent from the cache.

References

Copilot review on #1519 — 3 inline comments (_validation.py lines ~206, ~263, ~299).

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