[eas-cli] Auto-create branch when channel has no branches mapped

[gwdp]

eas update --channel <name> was failing with Channel has no branches associated with it when the channel existed but had no branch mapped.

This trips up fresh projects: eas build auto-creates the channel without a branch, so by the time you run eas update the channel already exists. The CLI only ever auto-created a branch when the channel was missing (ChannelNotFoundError), so the existing-but-empty channel fell through to the error instead.

Now the empty-channel case does the same thing as the missing-channel case: create a branch with the channel's name and map it.

Test plan

• Added a unit test for the empty-channel case
• Ran eas update --channel <empty channel> against a real project: the old CLI fails with the error above, this build creates the branch and publishes the update

[github-actions]

Subscribed to pull request

File Patterns	Mentions
**/*	@douglowder
packages/eas-cli/src/update/**	@wschurman, @quinlanj

Generated by CodeMention

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.03%. Comparing base (4071f48) to head (0953405).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3891      +/-   ##
==========================================
+ Coverage   59.01%   59.03%   +0.02%     
==========================================
  Files         935      935              
  Lines       40933    40931       -2     
  Branches     8624     8622       -2     
==========================================
+ Hits        24153    24158       +5     
+ Misses      16684    16677       -7     
  Partials       96       96              

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[github-actions]

✅ Thank you for adding the changelog entry!