Fix Edit Lambda for calls with omitted arguments

[jimmytacks]

Closes #151.

Summary

When Edit Lambda expanded a call like =EXPLODE(A1) where the LAMBDA declared more parameters (text, [chunk_size], [horizontal]) than the caller supplied, the omitted parameters were left as free references inside the body — making the generated LET invalid.

The fix:

• Detect the canonical IF(ISOMITTED(p), default, p) wrapper anywhere in the LAMBDA body for each omitted parameter p. Extract default and bind p, default at the top of the LET. Replace the IF wrapper with a bare p so the LET stays clean and round-trips through LET to LAMBDA without re-adding the wrapper unless the user re-checks "Optional".
• If no pattern is found for an omitted parameter (hand-written LAMBDA without the canonical pattern, or a missing required arg), fall back to binding p, "" so the LET is at least valid and the hole is visible to the author.
• Standalone ISOMITTED(p) references not wrapped in the IF pattern (e.g. Help?, ISOMITTED(text)) are left alone — when the param is provided in the call, ISOMITTED evaluates to FALSE on the bound name, which is the right semantics.
• The spec-0002 LetToLambdaBuilder wraps optional params as a LET binding named after the param (e.g. y, IF(ISOMITTED(y), 10, y)). After stripping, that becomes y, y — a no-op shadow of the outer binding. Drop those during flattening to keep the output clean.

Test plan

• dotnet build addin/lambda-boss.slnx clean (0 errors)
• dotnet test addin/lambda-boss.Tests/lambda-boss.Tests.csproj — 827/827 passing
• Added new tests covering: omitted optional with extracted default, multiple omitted optionals, default expressions containing nested commas, omitted optional inside an inner LET, standalone ISOMITTED on a provided param (not rewritten), IF(ISOMITTED) appearing inside a string literal (not rewritten), spec-0002 round-trip with the y, y shadow dropped, and missing-arg fallback to "".
• Manual smoke test in Excel against EXPLODE: call =EXPLODE(A1) then run /Edit Lambda — the produced LET should bind chunk_size, 1 and horizontal, FALSE with the IF wrappers stripped from _size/_horiz.

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