Related issue

Part of #1914. This PR handles the ClickHouse plugin.

Summary

• Add ClickHouseIdentifierProcessor as the plugin's identifier and string-literal processor. Conditional SPI quoting remains conditional, generated DDL uses an explicit always-quote path, embedded backticks round-trip correctly, and reserved-word matching is locale-independent.
• Route ClickHouse metadata, DDL builder, and manager paths through the dialect processor. Generated table names use ClickHouse's database.table form, prefer schemaName, and consistently qualify drop, truncate, and copy operations.
• Validate column types, table engines, and default expressions with quote-aware balanced scanners. Legal nested ClickHouse syntax such as Enum8('a' = -1), DateTime64(3, 'Asia/Shanghai'), Distributed(cluster, db, table, cityHash64(id)), and toDateTime64(now(), 3) is preserved without re-encoding serialized literals.
• Keep parameterized types in ALTER TABLE, emit valid rename/modify actions, and map schema create/drop operations to ClickHouse CREATE/DROP DATABASE.
• Add focused regression coverage for identifier round trips, qualified names, DDL generation, legal expressions, malformed expression rejection, and inherited builder paths.

Affected surfaces

• Frontend / Web
• Backend / API / Storage
• Database plugin / Driver
• JCEF / Desktop packaging
• CI / Build / Release
• Documentation only

Verification

• Commands and results: rtk mvn -B -pl chat2db-community-plugins/chat2db-community-clickhouse -am -f chat2db-community-server/pom.xml -Dmaven.test.skip=false -DskipTests=false -Dsurefire.failIfNoSpecifiedTests=false -Dmaven.test.failure.ignore=false clean test completed with BUILD SUCCESS across all 9 reactor modules. ClickHouse tests: 43 passed; MySQL dependency-module tests: 653 passed; failures/errors/skips: 0.
• Manual verification: exact generated SQL assertions cover embedded backticks, schema-qualified copy/drop/truncate, parameterized ALTER TABLE types, nested engine/default expressions, quoted literals, and rejection of unbalanced or multi-statement expressions.
• UI evidence: N/A

Risk and compatibility

• Public API or stored data: N/A. The change is isolated to generated ClickHouse SQL and the plugin's existing SPI implementation.
• Database or driver compatibility: legal ClickHouse type, engine, and default-expression syntax is preserved; names are emitted as backtick-quoted ClickHouse identifiers.
• Network, privacy, or security: no network or privacy behavior changes. Malformed metadata-derived SQL fragments are rejected instead of being emitted.
• Community / Local / Pro boundary: Community ClickHouse plugin only.
• Backward compatibility: ordinary object names remain compatible; schema-qualified DDL is now consistent, and previously malformed SQL for names containing backticks or parameterized types is corrected.

Reviewer map

• Start here: ClickHouseIdentifierProcessor, ClickHouseSqlGuards, ClickHouseSqlBuilder, and ClickHouseIdentifierProcessorTest.
• Failure condition: an identifier is double-quoted or loses embedded backticks, a legal nested ClickHouse expression is rejected or rewritten, or generated DDL omits/misqualifies a table name or parameterized type.
• Rollback or disable path: revert this PR; there is no runtime feature flag.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: Maintainer review, implementation refinements, test expansion, and PR description updates used AI coding assistance.