tidy occurrences of varname_leaves as well

[penelopeysm]

actually the horrific thing is that DynamicPPL.varname_leaves still works because in DynamicPPL there's a wholesale using AbstractPPL which brings it into scope. this PR is therefore not really fixing any error but just makes it look better

[github-actions]

Benchmark Report for Commit bf5cc90

Computer Information

Julia Version 1.11.6
Commit 9615af0f269 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

|                 Model | Dimension |  AD Backend |      VarInfo Type | Linked | Eval Time / Ref Time | AD Time / Eval Time |
|-----------------------|-----------|-------------|-------------------|--------|----------------------|---------------------|
| Simple assume observe |         1 | forwarddiff |             typed |  false |                  8.7 |                 1.6 |
|           Smorgasbord |       201 | forwarddiff |             typed |  false |                658.7 |                44.4 |
|           Smorgasbord |       201 | forwarddiff | simple_namedtuple |   true |                432.6 |                51.8 |
|           Smorgasbord |       201 | forwarddiff |           untyped |   true |               1155.1 |                31.0 |
|           Smorgasbord |       201 | forwarddiff |       simple_dict |   true |               6673.7 |                29.4 |
|           Smorgasbord |       201 | reversediff |             typed |   true |               1447.8 |                29.8 |
|           Smorgasbord |       201 |    mooncake |             typed |   true |               1021.0 |                 4.6 |
|    Loop univariate 1k |      1000 |    mooncake |             typed |   true |               5872.1 |                 4.3 |
|       Multivariate 1k |      1000 |    mooncake |             typed |   true |               1024.7 |                 8.8 |
|   Loop univariate 10k |     10000 |    mooncake |             typed |   true |              66048.8 |                 3.9 |
|      Multivariate 10k |     10000 |    mooncake |             typed |   true |               8911.9 |                 9.8 |
|               Dynamic |        10 |    mooncake |             typed |   true |                136.1 |                12.2 |
|              Submodel |         1 |    mooncake |             typed |   true |                 12.5 |                 5.7 |
|                   LDA |        12 | reversediff |             typed |   true |               1461.1 |                 1.9 |

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.89%. Comparing base (c8e5841) to head (bf5cc90).
⚠️ Report is 1 commits behind head on breaking.

Files with missing lines	Patch %	Lines
src/model_utils.jl	50.00%	1 Missing ⚠️
src/test_utils/sampler.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           breaking    #1031   +/-   ##
=========================================
  Coverage     82.89%   82.89%           
=========================================
  Files            39       39           
  Lines          3969     3969           
=========================================
  Hits           3290     3290           
  Misses          679      679           

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