feat(gen): derive mappings with caller-supplied parameters

[ernado]

Closes #376.

Problem

PR #374 dropped the Photo/InputPhoto → InputPhotoFileLocation mappings
because InputPhotoFileLocation requires a thumb_size field that can't be
derived from the source constructor. As #376 notes, the generator could instead
emit AsInputPhotoFileLocation(thumbSize string) and let the caller pass it in.

Change

mappableFields now splits the target's unmappable fields:

• conditional fields stay optional (left unset, as before);
• thumb_size on file-location inputs becomes a method parameter (parameterField);
• anything else still causes the mapping to be skipped (unchanged).

The generated concrete As-mappers render those fields as parameters and assign
them after the derived fields.

Generated API

// changed (thumb_size was previously left empty)
func (d *Document) AsInputDocumentFileLocation(thumbSize string) *InputDocumentFileLocation
func (i *InputDocument) AsInputDocumentFileLocation(thumbSize string) *InputDocumentFileLocation

// new (previously not generated)
func (p *Photo) AsInputPhotoFileLocation(thumbSize string) *InputPhotoFileLocation
func (i *InputPhoto) AsInputPhotoFileLocation(thumbSize string) *InputPhotoFileLocation

telegram/query/messages now uses these mappers instead of building the
location structs by hand.

Breaking: AsInputDocumentFileLocation() gains a thumbSize argument.
Pass "" to keep the previous behavior (full file, not a thumbnail).

Scope

The blast radius is exactly the four file-location mappers above — the
parameterized mappings carry a Constructor, so the slice/field wrapper
templates (which only handle constructor-less interface mappings) are untouched.

Tests

• gen: unit tests for mappableFields (thumb_size → parameter; unmappable
  required field still skipped) and parameterField.
• make check_generated clean; full build + telegram/query/messages race
  tests pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.10%. Comparing base (6ac0462) to head (570d001).

Files with missing lines	Patch %	Lines
gen/fields.go	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1771      +/-   ##
==========================================
+ Coverage   71.07%   71.10%   +0.03%     
==========================================
  Files         501      501              
  Lines       23500    23501       +1     
==========================================
+ Hits        16703    16711       +8     
+ Misses       5570     5569       -1     
+ Partials     1227     1221       -6     

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