Fix ty diagnostics from the crossed #833/#845 merges

[d-chambers]

Description

dev's lint job is currently broken: #833 (chunk-by-size) and #845 (ty-api-audit) each passed CI against pre-merge dev, but their combination fails pre-commit run ty with two diagnostics in dascore/utils/chunk_plan.py. Every open PR now fails lint on its merge ref until this lands, so this is a small unblock:

• get_byte_count(value) in the size-diagnostics block: value can be None in merge mode, but diagnostics are only collected on the size-chunk path where it is an information Quantity — narrowed with an assert.
• _output_dtypes: a scalar-returning apply on a SeriesGroupBy is always a Series, but the stubs claim DataFrame | Series — pinned with an annotated assignment and a ty: ignore[invalid-assignment] at the origin.

No behavioral changes.

Changelog

none

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings and/or appropriate doc page.
• included tests (covered by the existing chunk/spool suites; tests/test_core/test_patch_chunk.py and test_spool.py pass locally).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

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Run ID: 356eaed4-d393-4b23-bbd6-77222ef2e99c

📥 Commits

Reviewing files that changed from the base of the PR and between 28fa0b8 and c92fa34.

📒 Files selected for processing (1)

• dascore/utils/chunk_plan.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.98%. Comparing base (28fa0b8) to head (c92fa34).
⚠️ Report is 2 commits behind head on dev.

❌ Your project status has failed because the head coverage (99.98%) is below the target coverage (100.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #851   +/-   ##
=======================================
  Coverage   99.98%   99.98%           
=======================================
  Files         164      164           
  Lines       18191    18192    +1     
=======================================
+ Hits        18189    18190    +1     
  Misses          2        2           

Flag	Coverage Δ
network	48.30% <0.00%> (-0.01%)	⬇️
unittests	99.98% <100.00%> (ø)

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