fix(azure): collect Synapse recommendations in the provider fan-out

[cristim]

Refs #47 (wires Synapse; Search and Managed Redis are deliberately left out, see below, so #47 stays open until that is decided)

Stacked on #378 (base fix/292-azure-advisor-derive-fields). Merge order: #366, #378, this.

Per-client check

• synapse: complete. Valid Consumption resource type (SqlDataWarehouse), pricer, AppendConsumptionVariants, pagination cap, and 50 tests. Wired under ServiceDataWarehouse, covered by the new adapter test, and excluded when another service is requested.
• search: GetRecommendations is an intentional no-op. AzureSearch is not a valid Consumption resourceType, and fix(azure/search): Azure Cognitive Search has no reservation rows in the retail catalog, so quotes always error #197 shows the retail catalog has no Cognitive Search reservation rows, so quotes always fail. Wiring it would add nothing.
• managedredis: its Consumption filter is RedisCache, the same resource type the already-wired cache client queries, so wiring it would emit each Redis recommendation twice. Newer Azure_Managed_Redis_* SKUs are also unsupported (feat(azure): define newer Managed Redis SKU support across discovery and quotes #188). It needs a decision on how it splits from the cache client first.

Evidence

TestGetRecommendations_CollectsSynapseForDataWarehouse is fixture-based (fake client) and does not compile on the parent, which has no Synapse constructor hook. go test ./... in providers/azure passes; pre-commit gocyclo passes (the Advisor-fn fallback was extracted to keep GetRecommendations under the limit).

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[cristim]

Independent review (claude-opus-5-5) of 479eddb: APPROVE

Scope: this PR's own diff against its parent #378 (2673415), commits 57168a5 and 479eddb. Review only; no Azure calls were needed for this diff.

Evidence (local, fresh clone, macOS, fixture-based):

• Final tree at 479eddb: go vet ./... and go test ./... in providers/azure pass; GOWORK=off go build ./... passes for pkg, providers/aws, providers/azure and providers/gcp; gofmt -l clean; misspell (US locale) clean on the changed file.
• The intermediate commit 57168a5 is not what lands; the final tree builds and tests clean.
• Mutation: forcing includeSynapse to false makes TestGetRecommendations_CollectsSynapseForDataWarehouse fail, so the wiring test is not vacuous.
• Synapse results join mergeServiceResults as an attempted service (includeSynapse), so a Synapse failure counts toward the all-attempted-failed guard. The Synapse client returns an error on page errors and the page cap, so it never yields a silent partial.

Non-blocking:

1. providers/azure/recommendations.go:113 still says "All six service calls"; there are now seven goroutines.
2. Search and Managed Redis staying unwired is stated only in the comment at recommendations.go:195-196 ("see fix(azure): the recommendations aggregator never calls managedredis, search or synapse #47"); fine as long as fix(azure): the recommendations aggregator never calls managedredis, search or synapse #47 tracks it.

Gate notes (not findings against this diff):

• CI has not run on this head: ci.yml triggers on pull_request to main/develop and the base is the fix(azure): derive Advisor fields from the payload and fail loud on truncation #378 branch. Fresh green CI on the exact head is still required after retarget.
• This PR inherits the parent stack. My reviews of fix(azure): surface skipped consumption recommendations as a typed partial-result error #366 and fix(azure): derive Advisor fields from the payload and fail loud on truncation #378 found blocking issues (PartialResultError is unreachable outside the module and dropped by the multi-subscription merge), so this should merge only after those are fixed and this head is re-verified on top.

[cristim]

Independent review (claude-opus-5-5) of 87db9c8: APPROVE

Re-review against parent #378 (462df7e). Merge-base = 462df7e. Review only.

• The doc comment now reads "All seven service calls" (recommendations.go:113).
• Synapse is stubbed (newSynapseClientFn = newFakeFn(...), restored in cleanup) in the fan-out tests that used the real client.
• Network check: I made the real newSynapseClientFn panic and ran the whole package. Exactly one test still constructs it: TestRecommendationsClientAdapter_GetRecommendations_PropagatesContextCancellation (recommendations_test.go:344). That test runs every real client on a context canceled before the call, and the Synapse loop checks ctx.Err() before NextPage (services/synapse/client.go:155-157), so it returns without a request. No test reaches the network through Synapse. The same pattern for the other real clients in that test predates this PR.
• Evidence (local, fresh clone, macOS): go vet and go test ./... in providers/azure pass. Forcing includeSynapse false fails TestGetRecommendations_CollectsSynapseForDataWarehouse.
• CI has not run on this head, because the base is the fix(azure): derive Advisor fields from the payload and fail loud on truncation #378 branch. Fresh green CI on the exact head is required after retarget.

Non-blocking: if strict isolation is wanted, stubbing Synapse in PropagatesContextCancellation too would make the test independent of the client's ctx check.