fix(cli): Normalize boolean flags before parsing

[reneleonhardt]

Background

Legacy boolean syntax (-keepalive false) could be interpreted as a positional argument, causing inconsistent parsing and keepalive behavior.
Positional parsing consumed -keepalive and treated false as an argument, so later flags were ignored and text consumers could fail on binary stdout ('utf-8' codec can't decode byte 0xff in position 0).

What

• Normalize boolean CLI args before parsing in both global and attack command paths.
• Added unit tests for boolean-arg normalization and parsing edge cases.
• Added unit coverage for attack boolean parsing and version output.

Reproduction

go run . attack -keepalive false -http2 false -duration 1s -rate 1 -output /tmp/vegeta-results.bin

Verification

• go test ./...
• go test ./... -run '^(TestNormalizeBooleanFlagArgs|TestNormalizeBooleanFlagArgsForUnknownFlags|TestAttackCmdLegacyBooleanStyleUsesNormalizedBooleanParsing|TestAttackCmdAttachedBooleanStyleUsesNormalizedBooleanParsing|TestMainVersionPrintsBuildMetadata)$'

Checklist

• Git commit messages conform to community standards.
• Each Git commit represents meaningful milestones or atomic units of work.
• Changed or added code is covered by appropriate tests.

Developed with carefully directed, manually reviewed AI assistance.

Co-Authored-By: GPT-5.6 Sol codex@openai.com