fix(login): support --password-stdin for piped input

[rambhardwajj]

Description

Fixes an issue with harbor login --password-stdin when password is provided via piped input
Previously, the login flow relied on term.ReadPassword(...), which expects an interactive terminal (TTY). This caused failures or incorrect behavior when stdin was non-interactive (e.g., piped input).

This change ensures correct handling of both interactive and piped stdin, and also fixes unintended password modification caused by trimming whitespace.

• Fixes [bug]: support --password-stdin for piped input and preserve whitespace #943

Type of Change

Please select the relevant type.

• Bug fix
• New feature
• Refactor
• Documentation update
• Chore / maintenance

Changes

• Use GetSecretStdin(...) to handle both interactive and piped stdin input
• Detect non-TTY stdin and read password directly from input stream
• Preserve leading/trailing whitespace in passwords (avoid TrimSpace)
• Trim only newline characters (\r, \n)
• Add unit tests for piped input and whitespace preservation