fix: repair WQP_Metadata.site_info and streamstats.Watershed

[thodson-usgs]

Summary

Two long-standing correctness bugs in dataretrieval helper classes, each with a regression test. The fixes are independent and non-breaking.

WQP_Metadata.site_info was dead code

The @property was defined inside __init__, so it never existed on the class. Accessing the documented md.site_info attribute fell through to BaseMetadata.site_info, which raises NotImplementedError.

It now lives at class scope, reads from self._parameters, and returns the what_sites() lookup for whichever of sites / site / site_no the query supplied (or None if none was). This matches NWIS_Metadata.site_info.

streamstats.Watershed was unusable

Three bugs compounded:

• from_streamstats_json assigned the parsed fields to class attributes and returned the class object rather than an instance — so every call mutated shared state and handed back the same object.
• __init__ fetched a watershed but discarded the result, leaving instances empty.
• get_watershed(format="shape") silently fell through to returning a Watershed instead of a shape, and get_sample_watershed() returned the raw response rather than a Watershed.

After this change:

• from_streamstats_json builds an independent instance (via __new__, bypassing the network-fetching __init__) and populates it through a shared _populate helper.
• __init__ fetches and populates self.
• the unimplemented format="shape" raises NotImplementedError instead of failing silently.
• get_sample_watershed() requests format="object", so it returns a Watershed as its docstring claims.

Related to #260: this repairs the Watershed class and the silent format fall-through non-breakingly. The broader "drop format, always return Watershed" proposal in that issue remains a separate (breaking) decision.

Tests

• tests/wqp_test.py — site_info is an accessible property; returns None when the query named no site and routes to what_sites otherwise.
• tests/streamstats_test.py (new) — from_streamstats_json yields independent instances; format handling for object / geojson / shape.

Verified on the current main: the wqp and streamstats test suites pass and ruff is clean.

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