CC-8364 Allow containers without imageName

[ghostwriternr]

Container Apps have a namespace-level image. workerd models that as imageName, and currently requires it on every Container-enabled Durable Object namespace.

Container Instances do not have a namespace-level image. Worker code chooses the image on start(), and a full container snapshot is also a valid source. Local development cannot represent Instances today.

This change makes imageName optional. Namespaces that still set it keep no-argument start() behavior. Namespaces that omit it must pass an image or a full container snapshot to start(). Named images stay on ctx.container.images so Worker code can pass them to start(). They are not a replacement for imageName.

A full container snapshot replaces the image, so restore starts from the snapshot even when the namespace has a different imageName. Directory snapshots only overlay files, so they still need an image or full snapshot underneath.

Tests cover API validation, named-image and empty configurations, direct image selection, missing sources, directory snapshots as a non-source, and Docker-backed snapshot restore against a different imageName.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 49.59350% with 124 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.41%. Comparing base (bc591a4) to head (12ead33).

Files with missing lines	Patch %	Lines
src/workerd/server/tests/container-client/test.js	0.00%	96 Missing ⚠️
src/workerd/server/container-client.c++	24.00%	17 Missing and 2 partials ⚠️
src/workerd/api/container.c++	50.00%	2 Missing and 3 partials ⚠️
src/workerd/server/server.c++	57.14%	2 Missing and 1 partial ⚠️
src/workerd/api/container-test.c++	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7307      +/-   ##
==========================================
+ Coverage   37.31%   37.41%   +0.10%     
==========================================
  Files         811      811              
  Lines      253974   254182     +208     
  Branches    20105    20110       +5     
==========================================
+ Hits        94776    95114     +338     
+ Misses     147788   147624     -164     
- Partials    11410    11444      +34     

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