[improve][client] Use SHA-256 for generated reader subscription names

[david-streamlio]

Motivation

When a reader is created without an explicit subscription name, ReaderImpl/MultiTopicsReaderImpl generate one by hashing a random UUID with SHA-1 (DigestUtils.sha1Hex). The hash is not a security control — it just produces a short random identifier — but strict security providers reject SHA-1: under BC-FIPS in approved-only mode the call throws, so creating a reader without a subscription name fails on FIPS-configured JVMs.

This is one of a set of small preparatory cleanups for FIPS-restricted deployments; a broader PIP is being drafted separately.

Modifications

Replaced DigestUtils.sha1Hex(...) with DigestUtils.sha256Hex(...) in the generated-subscription-name paths of ReaderImpl and MultiTopicsReaderImpl. The generated names keep the same shape (reader- / multiTopicsReader- prefix + 10 hex chars) and entropy (40 bits of a hash over a random UUID). The names are ephemeral, used for non-durable subscriptions, and never persisted, so nothing is observable across versions.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage. (Existing reader tests, e.g. ReaderImplTest, exercise the generated-name path.)

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[Copilot]

Pull request overview

This PR improves Pulsar client compatibility with FIPS-restricted JVMs by removing SHA-1 usage from autogenerated reader subscription names, which can fail under strict security providers.

Changes:

• Switch ReaderImpl autogenerated subscription name hashing from SHA-1 to SHA-256.
• Switch MultiTopicsReaderImpl autogenerated subscription name hashing from SHA-1 to SHA-256 (keeping the same prefix + 10-hex-char shape).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pulsar-client/src/main/java/org/apache/pulsar/client/impl/ReaderImpl.java	Replace DigestUtils.sha1Hex(...) with DigestUtils.sha256Hex(...) for generated reader subscription names.
pulsar-client/src/main/java/org/apache/pulsar/client/impl/MultiTopicsReaderImpl.java	Replace DigestUtils.sha1Hex(...) with DigestUtils.sha256Hex(...) for generated multi-topic reader subscription names.