fix(cli): reject empty channel messages

[oletillmann]

Summary

• bound buzz messages send --content - reads from completely idle non-terminal stdin so an inherited open pipe cannot block indefinitely
• reject channel/forum message envelopes that contain neither non-whitespace text nor media
• preserve interactive terminal input, slow producers, and media-only messages

Why

An agent process can inherit an open stdin descriptor and invoke --content - without attaching a producer. The CLI currently waits forever for the first byte. If the descriptor is eventually closed, an empty message can continue to the signer and relay.

This change adds two independent protections:

1. Non-terminal stdin gets five seconds to produce its first byte. After that first byte, the CLI waits normally for the producer to finish; terminal stdin remains intentionally interactive.
2. CLI, SDK, and desktop message builders reject empty/whitespace-only envelopes unless media is attached.

Tests

• regression test with an open pipe whose writer stays alive without writing or closing
• delayed-producer test that waits three seconds before writing valid content
• literal-empty and immediate-EOF rejection tests
• non-empty stdin path test
• SDK and desktop builder tests, including media-only messages
• cargo test -p buzz-cli
• cargo test -p buzz-sdk
• cargo clippy -p buzz-sdk -p buzz-cli --all-targets -- -D warnings
• just desktop-tauri-test
• just ci