Improve error messages for unsupported __audit database operations

[CRZbulabula]

Description

Replace the misleading read-only error used for unsupported operations on the audit database with a more accurate message:

Apache IoTDB does not support this operation on database '%s'.

The message is shared by the tree-model and table-model authorization paths and is localized in both English and Chinese.

This PR also fixes the internal audit writer exemption in TreeAccessCheckVisitor. The check now compares INTERNAL_AUDIT_USER with the request username instead of the target path.

Affected operations

• Apply the new message to tree-model write, schema, template, TTL, and database deletion checks that previously reported the audit database as read-only.
• Apply the same message to unsupported table-model operations on __audit.
• Keep the existing reserved-name, grant/revoke, and AUDIT-admin query messages unchanged because they already describe their restrictions precisely.
• Preserve writes from the internal audit user while rejecting external writes to root.__audit.

Test coverage

• Add coverage for DELETE DATABASE root.__audit and template operations in IoTDBTemplateAuthIT.
• Update TTL coverage in IoTDBSeriesPermissionIT.
• Add coverage for DROP DATABASE __audit in IoTDBAuthenticationTableIT.
• Add a focused unit test for internal audit writes, external audit writes, and writes to ordinary paths.

Verified with:

• mvn spotless:apply -pl iotdb-core/datanode
• IoTDBTemplateAuthIT#manageDataBaseTest
• IoTDBSeriesPermissionIT#ttlOperationsTest
• git diff --check

---

This PR has:

• been self-reviewed.
• added comments explaining the intent where it was not obvious.
• added or updated unit and integration tests.
• been tested in a test IoTDB cluster.

---

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

• AccessControlImpl
• ITableAuthCheckerImpl
• TreeAccessCheckVisitor
• DataNodeQueryMessages
• TreeAccessTest

[sonarqubecloud]

Quality Gate failed

Failed conditions
E Reliability Rating on New Code (required ≥ A)

See analysis details on SonarQube Cloud

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[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.87%. Comparing base (f7be88e) to head (93d3d65).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...an/relational/security/TreeAccessCheckVisitor.java	10.00%	18 Missing ⚠️
...ne/plan/relational/security/AccessControlImpl.java	33.33%	2 Missing ⚠️
...lan/relational/security/ITableAuthCheckerImpl.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #18309      +/-   ##
============================================
+ Coverage     42.83%   42.87%   +0.04%     
  Complexity      374      374              
============================================
  Files          5362     5362              
  Lines        381409   381600     +191     
  Branches      49509    49553      +44     
============================================
+ Hits         163368   163623     +255     
+ Misses       218041   217977      -64     

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