fix(spinner): suppress ANSI output when stderr is not a terminal

[KooshaPari]

Summary

Fixes #3615

When forge -p is used or output is piped/redirected, the spinner thread writes ANSI escape codes (\r\x1b[2K) to stderr, polluting non-TTY output. This PR adds terminal detection so the spinner is only created when stderr is an actual terminal.

Changes

• Added std::io::IsTerminal import
• Added is_terminal: bool field to SpinnerManager, auto-detected in new() via std::io::stderr().is_terminal()
• start() returns None (no spinner thread) when !is_terminal
• Added force_terminal() test helper (cfg(test) only) to allow unit tests to exercise spinner logic in non-TTY environments
• Updated pause/resume test to use the test helper
• All 10 spinner tests pass; full cargo check clean

Test plan

• cargo check -p forge_spinner — 0 errors
• cargo test -p forge_spinner — 10/10 pass
• forge -p "hello" 2>/dev/null — no ANSI in stdout
• forge -p "hello" | cat — clean output

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