[All] Fix ~28 code scanning warnings (batch 2)

[dbrattli]

Summary

• Follow-up to [All] Fix ~45 code scanning warnings in Fable.Transforms #4357 addressing four additional categories of Ionide analyzer warnings across Fable.Transforms and Fable.Compiler
• IONIDE-002: Prefer postfix type syntax for seq, option only (per F# style guide — ResizeArray and other types keep prefix form)
• IONIDE-005: Use String.IsNullOrEmpty instead of = "" pattern matches
• IONIDE-007: Use list pattern instead of cons (Recursive :: [] → [ Recursive ])
• IONIDE-010: Combine List.filter |> List.map into List.choose, merge consecutive List.map |> List.map into single List.map

All changes are mechanical and do not affect runtime behavior. Both Fable.Transforms and Fable.Compiler build with 0 warnings, 0 errors.

Test plan

• dotnet build src/Fable.Transforms/Fable.Transforms.fsproj — passes
• dotnet build src/Fable.Compiler/Fable.Compiler.fsproj — passes
• CI: ./build.sh test javascript
• CI: ./build.sh test python

🤖 Generated with Claude Code

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	18
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_hash_set.py	3	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	1	Excluded