Don't write a beamignore all the time

[jamie-retool]

When using a known/static image ID, the SDK shouldn't attempt to write a .beamignore file to disk. The file sync path (and its .beamignore) is only relevant when building a new image from local files. Writing unconditionally breaks in read-only or restricted environments where no file sync is happening. This moves initIgnoreFile() so it only runs when no explicit ignore patterns are provided, like when a file sync is actually needed.

Additionally, when prepareRuntime did fail, it threw a generic "Failed to prepare runtime" with no indication of the actual cause (bad client, failed build, volume not ready, etc.) because stub.prepare() only returned false. Now each failure point stores the real error on stub.lastError, which gets surfaced in the exception, making failures diagnosable.

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Summary by cubic

Stop writing .beamignore when a static image or explicit ignore patterns are used, and surface real errors from runtime preparation so failures are diagnosable.

• Bug Fixes
  • Only create/read .beamignore when no ignore patterns are provided (i.e., when file sync is needed). Prevents failures in read‑only environments and with known image IDs.
  • Store errors on StubBuilder.lastError at each failure point and include the message in SandboxConnectionError. Makes build, sync, volume, and API errors visible.
  • Added tests to verify error surfacing for file sync and stub creation failures.

^{Written for commit 27747fb. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files