Dedupe symbol count fields in pyrefly coverage

[jorenham]

Summary

The ModuleReport and ReportSummary structs both had the same 8 n_{symbol} count fields, which were modified in collect.rs one by one in three functions. So if you wanted to add a new symbol counter, you'd have to do so in 5 places.

This dedupes these fields using he same #[serde(flatten)] trick used for the "slot" (typable) counts.

This is purely a refactor; so there are no behavior changes.

This shouldn't cause any merge conflicts with #3747.

Test Plan

Tests still pass.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[NathanTempest]

Thanks @jorenham for your continued refactoring and improvements in pyrefly coverage. Pulling the eight repeated count fields into SymbolCounts with #[serde(flatten)] mirrors the existing SlotCounts pattern perfectly, and I confirmed it's output-identical (2 files changed, zero snapshot diffs). The merge() helper cleanly replaces the manual summation, and the n_type_ignores preservation in filter_module_report_to_public is handled correctly. LGTM, importing and merging.

[meta-codesync]

@NathanTempest has imported this pull request. If you are a Meta employee, you can view this in D108620077.

[yangdanny97]

Review automatically exported from Phabricator review in Meta.

[meta-codesync]

@NathanTempest merged this pull request in e4ad5a5.