test(gsheet-sync): #204 pin CSV<->grid round-trip contract (11 edge-case tests)

[jsboige]

What

Secondaire of ai-01 dispatch bwpx5q (gate-safe #204 deep-queue). Pins the home-grown CsvToGrid / GridToCsv converters — the bridge between a Google Sheet download and a local CSV, and between a local CSV and a Sheets upload — with 11 edge-case contract tests. The invariant that matters most is round-trip symmetry (grid → csv → grid identity) on the multiline / quoted / ragged data that real Argumentum CSVs actually contain.

This is the 2nd pure target the dispatch offered ("CsvToGrid edge"). The converters are already pure (static CsvToGrid, instance GridToCsv — both plain string→grid / grid→string transforms), so there is no inline logic to extract: this PR is additive test coverage on the fragile CSV round-trip contract, matching the spirit of #204 (expand coverage on fragile contracts).

Two non-obvious behaviours now pinned

Reading the code, two things are easy to "clean up" into a regression. Both are documented in the test file's XML doc and pinned by tests:

1. CsvToGrid is configured with HasHeaderRecord = true, but the manual while (csv.Read()) loop returns the header as the first grid row (there is no separate ReadHeader() skip). This is deliberate: Sheets expects the header row inside the uploaded grid. A "cleanup" that switched to GetRecords<T>() or added an explicit header skip would silently drop row 0 and break every upload. (Pinned by RoundTrip_* identity + the existing CsvToGrid_PreservesHeaderRow.)

2. The read loop is bounded by csv.Parser.Count, which is the current record's field count (dynamic, per-row), not the header's. So ragged rows (rows with fewer/more fields than the header) are preserved without throwing MissingFieldException, and round-trip without padding or truncation. (Pinned by RoundTrip_PreservesRaggedRows.)

Tests — CsvGridConversionContractTests (11)

Parse direction (RFC 4180):

• CsvToGrid_ParsesQuotedFieldWithEmbeddedNewline — a newline inside a quoted field is part of that field, not a record separator (highest-risk CSV edge case for Argumentum descriptions/examples).
• CsvToGrid_ParsesEscapedDoubleQuotes — "" → ".

Round-trip symmetry (the sync invariant):

• RoundTrip_PreservesQuotedFieldWithEmbeddedNewline — multiline cell survives grid→csv→grid.
• RoundTrip_PreservesEscapedDoubleQuotes
• RoundTrip_PreservesEmptyAndCommaFields — empty cells + comma-forcing cells.
• RoundTrip_PreservesRaggedRows — rows of differing lengths.
• RoundTrip_SingleColumnCsv_NoCommas — degenerate single-column CSV.

Degenerate inputs (no NRE, no phantom rows):

• CsvToGrid_EmptyString_ReturnsEmptyGrid
• GridToCsv_EmptyOrNullGrid_ReturnsEmptyString
• GridToCsv_NullCells_BecomeEmptyStrings — pins the cell?.ToString() ?? "" guard (null → empty, not literal "null").
• CsvToGrid_TrailingNewline_DoesNotCreatePhantomRow — the terminator newline must not add an empty row.

Verification

• New class: 11/11 pass.
• Full suite on branch: 329 passed / 0 failed / 5 skipped (306 master baseline → 318 post-test(localization): #204 #216 field-mapping contract — pure extraction + 12 tests #504 → 329 with these 11). No regression.
• Diff: 1 new test file, 0 production change. No CSV, no config, no RowsetNb/rscount, no workflow/rules.

Scope notes

• Test-only / output-neutral → mergeable during the release gate per ai-01's gate-safe framing (the gate holds even output-neutral pipeline behaviour changes; this PR changes no behaviour at all).
• SyncSafetyChecker (the other dispatch candidate) already has 11 tests covering its thresholds/boundaries/zero-rows/custom-config — well-covered, lower value, so not touched.

Contributes to #204. Refs #193 (GSheetSync), #216.

🤖 Worker po-2024 — secondaire of dispatch bwpx5q.

[clusterManager-Myia]

[NanoClaw] LGTM — test-only, 1 file (+197)

12 contract tests pinning CSV↔grid round-trip symmetry for GSheetService. Covers the right edge cases: RFC 4180 embedded newlines, escaped quotes, ragged rows, null cells, trailing newlines. Class docstring documents two non-obvious fragile behaviours (header row preserved deliberately, dynamic field count bound). Clean test-only extraction.

No concerns.