fix: harden shared agent source permissions

[lyingbug]

Summary

• validate the exact shared-agent source space through organization membership, share relation, and source agent before using it as a runtime selector
• resolve shared-agent web-search readiness, model/KB/MCP resources, and attachment processing dependencies from the authorized source space
• propagate the source selector through chat, temporary attachments, knowledge APIs, mentions, and frontend cache keys
• enforce the read-only share contract inside the agent engine by filtering source-space Wiki mutation tools
• add regression coverage for exact-source resolution, source web-search configuration, and shared-agent write-tool filtering

Root cause

Shared-agent execution already switched several backend runtime resources to the source space, but adjacent paths used inconsistent authority:

• the frontend checked the receiving space's web-search providers, so a valid source-space provider appeared unavailable
• agent IDs alone were ambiguous when multiple spaces shared same-ID built-in agents
• the HTTP knowledge permission boundary did not prevent the agent engine from invoking source-space Wiki write tools
• temporary documents mixed attachment ownership with the space used to resolve parser, ASR, and VLM resources

Impact

Recipients can use the capabilities intentionally configured on a shared agent, including source-space web search and scoped knowledge resources, without receiving provider credentials. Shared-agent runs remain read-only for source-space Wiki content, and a client-supplied source selector cannot grant access without a matching verified share relation.

Validation

• GOCACHE=/tmp/weknora-gocache-shared-agent-publish go test ./internal/application/repository ./internal/application/service ./internal/handler ./internal/handler/session ./internal/middleware ./internal/router ./internal/types ./internal/container
• npx tsx --test src/utils/agentWebSearch.test.mjs src/components/Input-field.agent-switch.test.mjs (7/7)
• npm run build-only -- --mode development
• git diff --cached --check