Remove Azure Bucket public access from the docs and the code

[Pranav-error]

Replaces #2137, which took the opposite approach. @stefanprodan and @matheuscscp both said on that PR that the docs should be corrected rather than the anonymous path restored, since no Bucket provider works without authentication — so this does that instead.

Refs #2136.

Docs

Three claims removed from docs/spec/v1/buckets.md:

• "If no chain can be established, the bucket is assumed to be publicly reachable."
• The azure-public example Bucket, which had no secretRef and relied on that behaviour.
• "Note: that for a publicly accessible object storage, you don't need to provide a secretRef nor serviceAccountName."

That last one sat under the Service Account section and applied to all providers, not just Azure. I checked the others before removing it: gcp.NewClient uses a token source whenever no secret is present, and minio.NewClient always sets minioOpts.Creds. Neither has an anonymous path, which matches what you said. Say the word if you would rather I scope that removal to Azure only.

Code

The anonymous path was unreachable anyway. azureauth.NewTokenCredential never returns nil, so creds was never empty, chainCredentialWithSecret could not return the documented nil, nil, and azblob.NewClientWithNoCredential was dead. Removed the fallback, the dead nil return and the len(creds) > 0 guard so the code states the same thing as the docs.

Left alone, tell me if you want them

• withoutCredentials() in blob.go is unexported and only called from blob_test.go, so it is an anonymous path reachable only by tests. It is unused in production either way; happy to remove it and adjust the tests in this PR or a follow-up.
• Test_chainCredentialWithSecret still passes unchanged, since the chain is still always non-empty.

Verification

go build ./..., go vet ./internal/bucket/azure/ and go test ./internal/bucket/azure/ all clean.

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