[build-tools] add worker-side ssh session helper

[gwdp]

Why

The worker needs a piece that starts an SSH session on the build VM and keeps the worker alive while someone is connected.

How

New helper resolves the upterm client vendored in the worker package, dials the relay, reports the connection details back so the CLI can attach, and holds on an idle timer. Redialing is bounded by a shared budget so a flapping relay can't retry forever. Connection secrets are scrubbed from logs.

Test Plan

Unit tests cover the connection parsing, secret redaction, process-liveness detection, the bounded redial budget, and the idle hold. CI passes.

[linear-code]

ENG-24547

[gwdp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [eas-cli] add --ssh to workflow:run #4033
• [eas-cli] add workflow:ssh command #4032
• [worker] wire ssh session into build #4031
• [build-tools] add worker-side ssh session helper #4030 👈 (View in Graphite)
• [eas-build-job] add SSH_SESSION build phase #4029
• main

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[codecov]

Codecov Report

❌ Patch coverage is 99.36306% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (gwdp/eng-24547-eas-build-job-ssh@1277e59). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
packages/build-tools/src/utils/upterm.ts	98.92%	1 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##             gwdp/eng-24547-eas-build-job-ssh    #4030   +/-   ##
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  Coverage                                    ?   61.15%           
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  Files                                       ?      965           
  Lines                                       ?    42947           
  Branches                                    ?     9060           
===================================================================
  Hits                                        ?    26262           
  Misses                                      ?    16591           
  Partials                                    ?       94           

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[github-actions]

✅ Thank you for adding the changelog entry!