fix: return error instead of panic when decoding ParquetScan/AvroScan without features

[nam2ee]

Which issue does this PR close?

• Closes Decoding a ParquetScan/AvroScan physical plan panics when the feature is disabled, instead of returning an error #24197.

Rationale for this change

Explained in #24197.

What changes are included in this PR?

I just replaced the three feature-gate panic!s in datafusion/proto/src/physical_plan/mod.rs with not_impl_err!, following the existing ParquetSink convention in the same file. Also fixes "a Avro" to "an Avro" in the message.

Are these changes tested?

Checked the same feature combinations as the datafusion-proto features CI job and ran cargo test -p datafusion-proto. Manually verified with a no-default-features build that decoding panicked before and returns NotImplemented after. No regression test added because the error path only exists in feature-disabled builds, which CI only runs cargo check on.

Are there any user-facing changes?

No API changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.04%. Comparing base (abc5ce7) to head (18423b4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24198      +/-   ##
==========================================
- Coverage   81.05%   81.04%   -0.01%     
==========================================
  Files        1106     1106              
  Lines      382398   382398              
  Branches   382398   382398              
==========================================
- Hits       309938   309914      -24     
- Misses      54149    54168      +19     
- Partials    18311    18316       +5     

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[kumarUjjawal]

Thank you @nam2ee

LGTM!