pgw#991: a bare-hex blob address is REFUSED, not guessed into a namespace

[PaulFidika]

Closes the SDK half of the th#1641 pair. The hub route landed as 0ce09518; this is the address the SDK was building for it.

The defect

request_context._download_blob_by_digest promoted any digest lacking a : to blake3:<hex>, and its docstring asserted the server "indexes all CAS content by blake3 digest". Neither has been true since th#1303:

• the repo-CAS is sha256 — internal/storage/cas_paths.go says so in a comment naming the lane: "the repo-CAS read path no longer speaks blake3 at all";
• blake3 survives for exactly one consumer, the dataset-CAS.

So a caller holding a bare sha256 hex — the form hub listings and snapshot manifests quote — was silently addressed into the namespace its blob is not in, and the miss read as missing DATA rather than as a malformed address.

The fix: refuse, don't re-point

The guess is deleted, not changed to sha256:. Two live namespaces disagree on the algorithm, so promoting bare hex is a coin flip whichever constant it picks — and the hub does not offer that choice. storage.ParseDigest answers bare hex with:

cas ref: not algorithm-tagged (bare hex is refused; write "sha256:<hex>")

and validateDigestParam fronts it on every by-digest route, including getBlobContentByDigest. _parse_cas_digest is that function transcribed — algorithm-tagged, a supported algorithm (sha256/blake3, the exact content of casSupportedAlgos), hex of that algorithm's declared width, lowercased — so the worker refuses exactly what the hub refuses and accepts exactly what it accepts, and it happens before the socket is opened.

The refusal is typed by th#1259 provenance, like every other address fault:

origin	raises	maps to
REF_ORIGIN_PAYLOAD (caller)	BlobDigestMalformedError (blob_digest_malformed, a PayloadRefError)	JOB_STATUS_INVALID — a tenant's typo is never model-health evidence
REF_ORIGIN_PLATFORM	RuntimeError	fatal — the platform emitting a malformed address IS a statement about the release

Test

tests/test_blob_digest_qualified_pgw991.py runs the real fetch path against a real HTTP server whose by-digest route runs a transcription of storage.ParseDigest and answers 400 invalid_digest exactly where the hub does — so a regression that re-introduces the guess fails on the hub's own rule, not on a worker-side assertion about it. Parity is asserted in both directions over a table of addresses (nothing the hub accepts is refused; nothing it refuses is sent), and the rig records every address that reaches the wire, which is what proves a malformed one reaches nothing.

• RED, by restoring the old blake3: line under the new tests: 11 failed, 4 passed.
• GREEN with the fix: 15/15.

Gates (local; run in this worktree)

mypy clean (233 source files) · ruff check src/gen_worker clean · lint_http_timeouts, lint_unreached_surface, lint_config_reads all exit 0 · full tests/ + tests_v2/ run reported in a follow-up comment.

No version bump and no CHANGELOG.md edit — changelog.d/pgw991.md is this lane's fragment, folded in at the cut.