fix(tabs): avoid throwaway tab before explicit selection

[zyz619963502zyz]

Summary

• defer fresh pinned-tab creation when the first auto-launched command explicitly selects an existing tab
• preserve fresh-tab behavior for other first commands and existing binding recovery
• add a focused regression for tab_switch with an explicit tabId

Fixes #1986.

Validation

• cargo test --bin agent-browser explicit_first_tab_selection_does_not_need_a_fresh_pinned_tab -- --nocapture
• cargo test --bin agent-browser tab_binding -- --nocapture
• cargo test --bin agent-browser auto_launch -- --nocapture (10 passed, 3 ignored)
• cargo fmt -- --check

cargo clippy --bin agent-browser -- -D warnings is currently blocked by 19 pre-existing clippy::uninlined_format_args warnings in cli/build.rs; no unrelated baseline files were changed.

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