[rupayon123]

ReadLineWithOutput currently ignores bufio.Reader.ReadLine's isPrefix result. Input longer than its buffer is returned as several answers, which can split UTF-8 and shift the next prompt response.

Accumulate fragments until the physical line ends. An unterminated final line is returned once before EOF; real non-EOF input errors still propagate. Trimming, CRLF and the cached reader remain unchanged.

Validation with Go 1.26.0 on macOS arm64:

• Seven failing cases and four controls now pass, including exact 4096/8192-byte final lines and an actual io.Pipe error.
• Full suite: 263 passing leaf cases across 16 packages. Affected interact/internal race checks: 117 passing cases across six packages.
• Vet, build, gofmt, tidy/module-lock checks and Staticcheck pass.
• Native subprocesses with actual piped stdin and the independent boundary probes return complete answers followed by stable EOF.

Supplementary whole-repository race testing has an unchanged progress.TestMultiProgressAutoRefreshMarksDirty failure, reproduced narrowly on the pristine base. This patch changes only the input helper and its regressions. Hosted platforms and terminal-event backends remain separate from these stream checks.