fix(docs): expand inline markdown inside table cells

[chrischall]

Closes #608.

Root cause

InsertNativeTable inserted raw cell content via a single InsertText request, so **bold**, *italic*, `code`, and [links] inside a markdown table cell rendered as literal characters. Outside of tables the converter already runs `ParseInlineFormatting` and emits per-span `UpdateTextStyle` requests — this PR runs the same pipeline on table cells.

Change

Extracted the per-cell request construction into a pure helper:

```go
buildTableCellRequests(cellContent string, cellIdx int64, isHeaderRow bool) ([]*docs.Request, int64)
```

so the inline-formatting expansion is directly unit-testable without HTTP mocking. The header-row whole-cell bold is preserved, and the running cell indices in `updateIndicesAfter` now shift by the UTF-16 length of the stripped text rather than the original markdown (since markers are gone from the inserted text).

Tests

• `TestBuildTableCellRequests_AppliesInlineBold` — `Alice` → InsertText "Alice" + UpdateTextStyle{Bold} on [cellIdx, cellIdx+5].
• `TestBuildTableCellRequests_AppliesInlineItalicAndCode` — covers `italic` and ` + "code" + `.
• `TestBuildTableCellRequests_HeaderRowAppliesBoldOverWholeCell` — preserves the existing header-bold behaviour.
• `TestBuildTableCellRequests_PlainTextNoStyleRequest` — guards against spurious style requests for plain cells.
• `TestBuildTableCellRequests_EmptyAfterStrippingReturnsNothing` — a marker-only cell is skipped cleanly without sending an empty batch.

`go test ./internal/cmd/ -count=1` → ok 24.2s.