Adds end-to-end coverage for the two graceful-degradation paths that were
previously only exercised by mock-based unit tests:

- TestStartFallsBackToLocalImageWhenPullFails: tags a real LocalStack
  image under an unpullable name so the pull fails but the image exists
  locally, and asserts lstk warns and starts the local image.
- TestStartContinuesWhenLicenseServerUnreachable: points the license
  endpoint at a closed server so the pre-flight fails at the transport
  level, and asserts lstk skips the check and the container still starts.

Both require Docker and a valid LOCALSTACK_AUTH_TOKEN (the container must
activate to become healthy), mirroring TestStartCommandSucceedsWithValidToken.

Refs DEVX-703

Co-authored-by: linear-code[bot] <222613912+linear-code[bot]@users.noreply.github.com>

On Ctrl+C mid-pull, the cancelled context made the rt.ImageExists
local-image probe fail, so the start surfaced a misleading "Failed to
pull" error and emitted a spurious start-error telemetry event. Guard the
pull-failure path with ctx.Err() so a user cancel propagates cleanly,
mirroring the existing license pre-flight handling.

Also documents the known limitation that an HTTP error response from the
license server (5xx, or 407 from a proxy) is still treated as a definitive
verdict rather than degrading.

Refs DEVX-703

Co-authored-by: linear-code[bot] <222613912+linear-code[bot]@users.noreply.github.com>

A pinned image that is already present locally is not pulled (pullImages),
so the CLI license pre-flight is now skipped for it too: the redundant
network round-trip would otherwise block an entirely offline start, and the
container validates its own bundled license at startup. This is symmetric
with the existing skip-pull behaviour for local pinned images.

tryPrePullLicenseValidation gains the runtime so it can probe ImageExists;
a probe error is non-fatal and falls through to the pre-flight check.

… path

The two offline start tests started a real container under an isolated
t.TempDir() HOME, so the container's root-owned volume files (e.g.
server.test.pem.key) could not be removed by t.TempDir cleanup, failing in
CI. Use the real (inherited) HOME like every other fresh-start container
test; config stays isolated via --config.

Adds TestStartSkipsPullAndLicenseCheckWhenImageIsLocal covering the #325
review's success path: a pinned configured image found locally starts with
no pull and no CLI license check (asserted via a license server that fails
the test if contacted).