fix(tools): migrate legacy community config files on startup

[HandSonic]

Related issue

N/A — regression found by reviewing the #2732 runtime-identity split; described below.

Summary

Upgrades from 4.0.0 silently reset the runtime config (systemUuid, networkStatus, latestStartupSuccessVersion) and minted a fresh client UUID: ConfigUtils only looks for the current runtime_config_{env}.json / client_uuid names, while 4.0.0 wrote enterprise_config_{env}.json / enterprise_client_uuid in the same directory, and nothing referenced the old names anymore. The client localStorage keys got the same treatment deliberately in a2333f5 — the server-side files were left orphaned.

Fix: one-time legacy migration where the files are resolved — when the current file is absent, a legacy sibling is copied to the current name (legacy files kept untouched as backup; copy via java.nio with logged, non-fatal error handling). Legacy names come from new default methods on ProductRuntimeIdentity (legacyRuntimeConfigFileName/legacyClientIdFileName) so other runtimes can override. No behavior change when the current file exists or neither exists.

Affected surfaces

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

Verification

• mvn -pl chat2db-community-tools -am test -Dmaven.test.skip=false: 72/72 green, incl. 4 new ConfigUtilsLegacyFileMigrationTest cases (copy fidelity, never-overwrite, no-legacy no-op, parent-dir creation).
• Fork CI green: fix(tools): migrate legacy community config files on startup HandSonic/Chat2DB#44.

Risk and compatibility

• Public API or stored data: additive only — reads legacy files, never deletes them.
• Database or driver compatibility: N/A.
• Network, privacy, or security: N/A.
• Community / Local / Pro boundary: the ProductRuntimeIdentity defaults return the community legacy names; a product whose legacy names differ overrides the default methods.
• Backward compatibility: full — downgrades still find the legacy files untouched.

Reviewer map

• Start here: ConfigUtils static block + migrateLegacyFile.
• Failure condition: unreadable legacy file → logged warning, default config written (same as today).
• Rollback or disable path: revert single commit; legacy files were never modified.

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: substantial — fix and tests drafted with AI assistance, verified locally.