Quiet expected teardown logs and lock capability reads

[vavo]

What does this pull request do?

Reduces expected teardown noise and removes unsynchronized external capability map reads.

Specific updates

• Demote cancellation-driven FFmpeg, segmentation, subscriber preconnect, and trickle client-disconnect messages from error-shaped output to debug-level logging.
• Add locked external capability lookup and name snapshots.
• Replace production direct map reads and protect mutable capability fields used by those readers.
• Add regression coverage for canceled trickle clients and concurrent capability access.
• Add pending changelog entries.

How did you test each of these updates?

• go test ./trickle -race -count=1
• Hosted CI is configured to run ./test.sh and ./test_e2e.sh.
• The local full-package run was blocked before Go tests by the macOS Homebrew FFmpeg 8 headers being incompatible with the pinned Livepeer FFmpeg bindings. The first local Docker bootstrap also failed while building its native x264 dependency; no repository files were changed by that attempt.

Does this pull request close any open issues?

Fixes livepeer#3922
Fixes livepeer#3885

Checklist

• I have read the contributing guide
• I have run the relevant tests
• I have added a pending changelog entry

[vavo]

Superseded by upstream draft PR livepeer#4010; keeping the branch available as the contribution head.