fix(dotnet): stop prepending raw stdout on build failure (#914)

[ousamabenyounes]

Summary

Fixes #914

• On dotnet build failure, RTK was prepending the full raw stdout before the filtered summary — producing more tokens than the original command
• The filtered summary from format_build_output already contains all errors and warnings; the raw output is redundant
• On success, only the filtered summary was shown (correct). This fix makes failure consistent with success

Verification

• Baseline tests: 1350 pass, 0 pre-existing failures
• Post-fix tests: 1351 pass, 0 regressions
• New test: test_format_build_output_on_failure_is_more_concise_than_raw verifies ≥60% token savings and error info preserved
• Review agent: issue alignment verified

Files changed

File	Change
src/cmds/dotnet/dotnet_cmd.rs	Remove raw-prepend block on failure; always use filtered; add savings test

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