Fix some unexpected behavior for thrift field mask.

[shenyj3]

Motivation

Fix some unexpected behavior for thrift field mask.

Required fields gated by field mask

Pilota-generated code checks _field_mask.field(id) for ALL fields, including required
ones. If the field mask selects $.some_struct.optional_field but not $.some_struct.required_field, the required
required_field field is omitted from the output.

Scalar not treated as all fields while map encoding

set_field_mask(fm) propagates a scalar (leaf) FieldMask to each map value struct's
_field_mask field. A scalar FM has no field-level children, so all inner fields are gated and NOT serialized.

Solution

1. Do not check existence of required field mask while encoding.
2. Treat Scalar as all fields.

[codecov]

Codecov Report

❌ Patch coverage is 80.20566% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.89%. Comparing base (7c30b6e) to head (d4cacf2).

Files with missing lines	Patch %	Lines
pilota-build/src/codegen/thrift/ty.rs	49.61%	65 Missing ⚠️
pilota-build/src/codegen/thrift/mod.rs	20.00%	8 Missing ⚠️
pilota-thrift-fieldmask/src/fieldmask.rs	98.40%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #362      +/-   ##
==========================================
+ Coverage   67.50%   67.89%   +0.39%     
==========================================
  Files          88       88              
  Lines       25044    25404     +360     
==========================================
+ Hits        16905    17249     +344     
- Misses       8139     8155      +16     

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