Fix race condition in collaborative session startup

[midigofrank]

Description

Two channel joins for the same workflow could both pass the lookup_shared_doc/1 fast-path and invoke start_document/2 concurrently. The second DynamicSupervisor.start_child returned
{:error, {:already_started, pid}}, which the strict {:ok, _} match did not handle, crashing the channel join.

Treat {:already_started, pid} as success in start_document/2.

Closes #4802

Validation steps

This is hard to replicate manually, I got it on tests though.

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[github-actions]

The PR is a small race-condition fix in collaborative session startup. Changes:

• lib/lightning/collaboration.ex — handle {:error, {:already_started, pid}} from SessionSupervisor.start_child/1 and return {:ok, pid} for idempotency
• test/lightning/collaborate_test.exs — new idempotency test
• CHANGELOG.md — entry

None of S0/S1/S2 are in scope: no project-scoped queries, no new web-layer actions, no config writes.

Security Review ✅

• S0 (project scoping): N/A — change is internal supervisor child-start logic in Lightning.Collaborate.start_document/2; no new queries or web entrypoints touching project-scoped data.
• S1 (authorization): N/A — no new controller/LiveView/channel actions; existing channel join authorization is unchanged.
• S2 (audit trail): N/A — no writes to workflows, credentials, settings, or other config resources.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.3%. Comparing base (6845844) to head (a1c21f3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4803   +/-   ##
=====================================
  Coverage   90.3%   90.3%           
=====================================
  Files        442     442           
  Lines      22543   22545    +2     
=====================================
+ Hits       20353   20356    +3     
+ Misses      2190    2189    -1     

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