#34058 acpmux: thread result_line through bg_start_with_id (base compile red)

Fixes a base compile red. feat-cmux-next stopped compiling the acpmux library at hub/bg_tasks.rs:160 (cannot find value result_line in this scope). Every fleet Release compile from 7da5453c onward has failed with it, including PR heads that merge the current base.

Cause: two PRs that each compiled on their own. #33779 moved the body of bg_start into a new bg_start_with_id, which has no result_line parameter. #33347 added result_line to that body. After both landed, bg_start takes result_line but never passes it on.

Fix: bg_start_with_id takes result_line and bg_start passes it through. The workflow caller (workflow_activity.rs) passes false, which is what its launch got before #33347. This is the minimal fix, so it also carries #[allow(clippy::too_many_arguments)], as other hub methods do.

cmux-ci on fleet Linux: cargo clippy -p acpmux --lib -D warnings is clean, and cargo test -p acpmux --lib -- bg_ workflow has 103 passed, 0 failed.
Agent session: agent session

🤖 Generated with Claude Code

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Summary by cubic

Fixes the acpmux compile red on feat-cmux-next by threading result_line through bg_start_with_id.

Two PRs that each compiled on their own broke the build after landing: one moved the body of bg_start into bg_start_with_id without a result_line parameter, and the other added result_line to that body. The fix gives bg_start_with_id a result_line parameter and has bg_start pass it through. The workflow caller in workflow_activity.rs passes false, matching its behavior before the parameter was introduced. Clippy and bg workflow tests pass locally.