[Test] Improve subscription client and disabled-mode coverage

[Caideyipi]

Description

Subscription protocol payload coverage

• Cover poll request and response serialization for all payload types, unknown type lookup, null progress, and termination responses.
• Cover subscribe RPC request/response conversion, including handshake, subscribe, unsubscribe, commit, poll, close, seek, empty bodies, sparse bodies, and null/empty poll buffers.

Client payload and utility coverage

• Cover record, TsFile, and watermark message handlers, incompatible handler access, cleanup, copy/move/delete lifecycle, and poll result defaults.
• Cover identifier parsing, set partitioning, and poll timer expiration, monotonic updates, overflow, and invalid timeout handling.

Disabled-mode integration coverage

• Add an integration test for the community build where subscription is disabled.
• Verify topic/subscription session statements return SUBSCRIPTION_NOT_ENABLED_ERROR.
• Verify consumer handshake exposes the expected connection exception and critical exception chain.

Verification

• iotdb-client/subscription full unit test suite: 84 tests passed.
• IoTDBSubscriptionDisabledIT: 2 tests passed with the default integration-test profile.
• IoTDBSubscriptionDisabledIT: 2 tests passed with the ClusterIT profile.
• Spotless, Checkstyle, and git diff --check passed.
• Checked open subscription-related PRs; no changed test file overlaps were found.

No new product defect was found while adding and running these tests.

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This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

• org.apache.iotdb.rpc.subscription.payload.poll
• org.apache.iotdb.rpc.subscription.payload.request
• org.apache.iotdb.rpc.subscription.payload.response
• org.apache.iotdb.session.subscription.payload
• org.apache.iotdb.session.subscription.util
• org.apache.iotdb.session.it.IoTDBSubscriptionDisabledIT