feat(benchmarks): enable Criterion automatically when a benchmark namespace is set

[Omega359]

Is your feature request related to a problem or challenge?

Running a SQL benchmark with --criterion-namespace or BENCH_NAMESPACE currently requires a separate --criterion flag. Since namespaces apply to Criterion benchmark groups, selecting a namespace should also select Criterion mode.

Describe the solution you'd like

Enable Criterion when either namespace option is set. Apply the existing mode checks after inferring Criterion mode, so incompatible --output and --iterations options still fail and --save-baseline works. Print the inferred-mode notice to stderr to preserve stdout output.

Describe alternatives you've considered

Keeping the explicit --criterion requirement would preserve the current behavior but require callers to specify both options.

Additional context

This follows the SQL benchmark migration preparation in #24806.

Willingness to contribute

Implementation included in this draft PR.

Which issue does this PR close?

• Part of [EPIC] Add SQL based benchmarking #21706.
• Prepares for Update bench.sh to use the new benchmark_runner #24311 and Remove old dfbench runner once benchmarks are migrated to the new SQL benchmark framework #24312.
• This PR does not close these issues.

Rationale for this change

Allow benchmark callers to select namespaced Criterion runs with a single option or environment variable.

What changes are included in this PR?

• Infer Criterion mode from --criterion-namespace and BENCH_NAMESPACE and update the CLI help.
• Share namespace validation between the CLI and the Criterion runner.
• Extend CLI tests for inferred mode, environment defaults, CLI overrides, list and dry-run behavior, conflicting options, saved baselines, and stderr notices.

What is the testing strategy for this PR?

The benchmark-runner tests exercise argument parsing and environment handling, including a subprocess test for BENCH_NAMESPACE. Existing tests cover invalid namespaces and distinct Criterion artifacts.

Are there any user-facing changes?

Yes. --criterion-namespace and BENCH_NAMESPACE now enable Criterion without an explicit --criterion flag. The runner reports inferred mode on stderr.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.34146% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.80%. Comparing base (454ca08) to head (c89713c).

Files with missing lines	Patch %	Lines
benchmarks/src/bin/benchmark_runner.rs	96.29%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26210      +/-   ##
==========================================
- Coverage   82.80%   82.80%   -0.01%     
==========================================
  Files        1149     1149              
  Lines      452213   452272      +59     
  Branches   452213   452272      +59     
==========================================
+ Hits       374473   374520      +47     
- Misses      54945    54953       +8     
- Partials    22795    22799       +4     

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