fix(dashboard): stop swallowing build-page load errors as 'not found'

[santhiprakash]

Why

On /build/:id, a successful deployment can render the ResourceNotFound screen (#604). The route is fine — the page's own notFound state is reached because loadBuildSession collapses every failure into { success: false }, and the page has exactly one branch for that. So any client-side exception during hydration (the reporter's 3-service projectType: "services" deploy, or any other throw inside the ~200-line restore) presents an existing, ready deployment as "not found".

That defeats an invariant the API deliberately establishes (deployment.controller.ts): a genuine miss is a 404 and everything else is surfaced as a 500 precisely so it doesn't get swallowed as a UI "not found" — and the client then swallows its own exceptions into exactly that UI.

What changed

• loadBuildSession now distinguishes the two failure modes. A soft-failed status response or an HTTP 404 returns notFound: true (genuine miss); a hydration exception, a 5xx, or a network failure returns notFound: false with the error message. Classification lives in a new pure helper (classifyBuildSessionFailure) so it's unit-testable.
• The build page renders the non-not-found arm as an error state with a retry (reusing the existing chrome.error.* copy and the shared ResourceNotFound panel), instead of "Deployment not found". A retry re-runs the load for the same deployment id.
• An empty composeServices array no longer blanks an already-loaded service list. The API ships composeServices: [] for services-type deployments that report their services through services/serviceStatuses instead; the old Array.isArray(...) guard replaced the loaded list with []. Empty now falls back to what's already loaded, same as the absent case always did (hydrateSnapshotServices).

Verification

Test that fails without the change (load-session.test.ts, 9 new tests):

cd apps/dashboard && bun run test src/context/deployment/load-session.test.ts

• hydrateSnapshotServices([], loaded) keeps the loaded list — with the previous inline code, the empty array produced [] (the reported blanking).
• classifyBuildSessionFailure marks only ApiError(404) as not-found; a 500, a hydration TypeError, and a network failure all classify as load errors.

Full local verification on upstream main (00326a2e):

bun install --frozen-lockfile
cd apps/dashboard
bun run test    # 80 files, 977 tests passed (includes the 9 new)
bun run lint    # tsc --noEmit, clean

bun format was run on the touched files; the two pre-existing formatting-drift hunks in files this PR doesn't restructure were left untouched.

Fixes #604