Fix potential panic bugs in string slicing and server shutdown

[hobostay]

Summary

This PR fixes several potential panic bugs and race conditions found in the codebase:

1. Race Condition in Desktop Server Shutdown (crates/openfang-desktop/src/server.rs)

Problem: Both shutdown() and Drop implementation tried to send shutdown signals and join threads, which could lead to race conditions when both try to manipulate the same resources simultaneously.

Fix:

• Added AtomicBool flag (shutdown_initiated) to track shutdown state
• Both shutdown() and Drop now use compare_exchange to ensure shutdown only happens once
• This prevents duplicate shutdown attempts and potential race conditions

2. String Slicing Panic (crates/openfang-api/src/channel_bridge.rs)

Problem: Multiple locations used &str[..8] to truncate UUID strings for display without checking if the string was at least 8 characters long. While UUIDs are always 36 characters, this pattern is unsafe and could cause panics if the data changes.

Fix:

• Added truncate_str() helper function that safely returns the full string if it's shorter than the requested length
• Fixed 9 occurrences at lines: 354, 393, 408, 431, 491, 513, 545, 565, 609

3. String Slicing Panic (crates/openfang-runtime/src/tool_runner.rs)

Problem: Canvas filename generation used &canvas_id[..8] without checking length.

Fix:

• Added length check before slicing
• Falls back to full string if shorter than 8 characters

Test Plan

• Build passes: cargo build --workspace
• Tests pass: cargo test --workspace
• No new clippy warnings introduced
• Manual code review of all changes

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

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