Session private drop with env

[browndav-msft]

Enables configuring the blob storage session (SessionMode and session container name) through environment variables / Configuration, so callers can opt into a session without changing builder code.

Two new keys are introduced in BuilderHelper: - AZURE_STORAGE_SESSION_MODE — names a SessionMode (NONE, AUTO, SINGLE_SPECIFIED_CONTAINER), matched case-insensitively. Only applied when the builder is still on the default SessionMode.AUTO.

AZURE_STORAGE_SESSION_CONTAINER_NAME — supplies the container name to scope the session to. Only applied when the builder has no container name set.
A new helper, BuilderHelper.applyEnvironmentSessionDefaults(SessionOptions, Configuration, ClientLogger), performs the fallback resolution and is invoked from:

BlobClientBuilder (both pipeline construction paths)
BlobContainerClientBuilder (both pipeline construction paths)
BlobServiceClientBuilder#constructPipeline
Explicit programmatic configuration on the builder always wins; the env vars are only consulted to fill in unset values. An invalid AZURE_STORAGE_SESSION_MODE value throws IllegalArgumentException via the client logger.

[Copilot]

Pull request overview

This PR enables turning on Storage Blob “session” authentication via environment variables / Configuration without requiring code changes to explicitly set SessionOptions. It also corrects session signing to normalize Content-Length: 0 consistently with Shared Key and server canonicalization, with updated regression tests.

Changes:

• Added BuilderHelper.applyEnvironmentSessionDefaults(SessionOptions, Configuration, ClientLogger) and new configuration keys AZURE_STORAGE_SESSION_MODE and AZURE_STORAGE_SESSION_CONTAINER_NAME.
• Wired env/config fallback into BlobClientBuilder, BlobContainerClientBuilder, and BlobServiceClientBuilder pipeline construction.
• Updated session signing and tests to normalize Content-Length: "0" to "", and added/updated tests around env-based activation.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/implementation/util/BuilderHelper.java	Adds env/config-driven defaults for SessionOptions (new keys + parsing/validation).
sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/BlobServiceClientBuilder.java	Applies env/config session defaults before building the pipeline.
sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/BlobContainerClientBuilder.java	Applies env/config session defaults prior to session validation and pipeline construction.
sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/BlobClientBuilder.java	Applies env/config session defaults in both sync/async build paths.
sdk/storage/azure-storage-blob/src/main/java/com/azure/storage/blob/implementation/util/StorageSessionCredential.java	Normalizes Content-Length: 0 to empty for session string-to-sign (align with Shared Key/server).
sdk/storage/azure-storage-blob/src/test/java/com/azure/storage/blob/BuilderHelperTests.java	Adds unit tests for env/config-driven session activation and precedence rules.
sdk/storage/azure-storage-blob/src/test/java/com/azure/storage/blob/ContainerApiTests.java	Adds disabled live end-to-end test for env-var activation (sync).
sdk/storage/azure-storage-blob/src/test/java/com/azure/storage/blob/ContainerAsyncApiTests.java	Adds disabled live end-to-end test for env-var activation (async).
sdk/storage/azure-storage-blob/src/test/java/com/azure/storage/blob/implementation/util/SessionTokenCredentialPolicyTest.java	Updates regression test to assert Content-Length: 0 normalization behavior.
sdk/storage/azure-storage-blob/src/test/java/com/azure/storage/blob/implementation/util/StorageSessionCredentialTest.java	Updates test commentary to reflect the corrected Content-Length behavior.