[fix][broker] Enable shadow topics by default

[zymap]

Motivation:
PR #26739 introduced enableShadowTopics with a false default, disabling existing shadow-topic deployments unless operators explicitly enabled it. Default the new setting to true to preserve the previous behavior.

Modifications:

• Enable shadow topics by default in ServiceConfiguration and the broker and standalone configuration files.
• Cover the default and explicit false override in configuration loading.
• Run ShadowTopicTest with the default setting and use a dedicated broker with the setting explicitly disabled in ShadowTopicsDisabledTest.

Validation:

• All 31 tests in PulsarConfigurationLoaderTest, ShadowTopicTest, and ShadowTopicsDisabledTest passed with retries disabled.
• Related Checkstyle checks and git diff --check passed.
• Full quickCheck was blocked by an existing missing ASF license header in pulsar-client's generated NasaMission.java.

Assisted-by: OpenAI Codex

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

Motivation

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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

If the box was checked, please highlight the changes

• 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

[codelipenghui]

LGTM. The change is minimal and correct, and the focused tests pass locally (ShadowTopicTest, ShadowTopicsDisabledTest, PulsarConfigurationLoaderTest; 31/31 with -PtestRetryCount=0).

Two follow-ups:

• branch-5.0 still has enableShadowTopics = false, and 5.0.0 shipped with it. Please add release/5.0.1 as well. Otherwise an upgrade from 4.2.6 or 4.0.15 to 5.0.x disables shadow topics again.
• This reverses the default that #26739 shipped in 4.0.14, 4.2.5 and 5.0.0. Please make sure @lhotari agrees, and add a release note (e.g. release/important-notice) so that users who rely on the disabled default know to set enableShadowTopics=false explicitly.

[lhotari]

This is intentionally disabled.

[codelipenghui]

@lhotari any reason to disable this feature? is there a proposal before?