Make AI conversation cold-stage reads opt-in

[wu-sheng]

Fix AI conversation queries implicitly reading cold storage

When BanyanDB cold storage was configured, opening an AI conversation or exporting its raw files queried both the default stages and cold, even when the caller had not selected cold. Every head-round, round-window, and session-data read now issues one query: omitted/false leaves the stage clause absent, while explicit coldStage=true emits ON cold STAGES.

The HTTP view accepts an optional coldStage parameter, and raw-file GraphQL queries accept ConversationCondition.coldStage. The flag is carried through every read; there is no automatic fallback to another stage. The list continues to use Duration.coldStage. Other storage implementations ignore the flag.

Uses the merged API from apache/skywalking-query-protocol#174; the protocol submodule points to merged commit 35ae8f2a7662df3d0b9681f635f24bf4e3f23d28.

• Add unit tests to verify the fix.
• Explain why the bug exists and how it is fixed.
• Update documentation and the CHANGES log.

Validation: JDK 17 reactor build covering the conversation module, GraphQL, and all three storage implementations; 22 focused tests passed. Tests cover omitted/false/true HTTP flags, view/export propagation across all windows, no fallback on a missing conversation, and emitted BanyanDB stage clauses with cold storage configured.