fix: Fix the grammar for constant function terms

[zrho]

This PR fixes the PEST grammar for constant function terms. It erroneously expected a nested term instead of a nested region. Regression tests are included.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.14%. Comparing base (0b091e2) to head (abf57cd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2139      +/-   ##
==========================================
+ Coverage   83.09%   83.14%   +0.05%     
==========================================
  Files         219      219              
  Lines       41918    41910       -8     
  Branches    37993    37985       -8     
==========================================
+ Hits        34830    34846      +16     
+ Misses       5278     5252      -26     
- Partials     1810     1812       +2     

Flag	Coverage Δ
python	85.70% <ø> (ø)
rust	82.88% <ø> (+0.05%)	⬆️

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[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure feature_missing: package feature removed or renamed ---

Description:
A feature has been removed from this package's Cargo.toml. This will break downstream crates which enable that feature.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/feature_missing.ron

Failed in:
feature model_unstable in the package's Cargo.toml
feature hugr-model in the package's Cargo.toml

--- failure feature_missing: package feature removed or renamed ---

Description:
A feature has been removed from this package's Cargo.toml. This will break downstream crates which enable that feature.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/feature_missing.ron

Failed in:
feature hugr-model in the package's Cargo.toml
feature model_unstable in the package's Cargo.toml