feat(flow-chat): expose retry attempt diagnostics

[wsp1911]

Summary

• Capture retry diagnostics for transient provider failures, empty effective output, and invalid tool arguments.
• Preserve original provider errors, raw tool JSON, and parser or validation errors through the event and session-history contracts.
• Show historical retry attempts in Flow Chat with an on-demand, copyable diagnostic disclosure.
• Immediately supersede abandoned attempts in the live UI so active tool cards are cancelled while the round continues retrying.

Type and Areas

Type: Feature

Areas: Rust core, agent stream, event contracts, session persistence, web UI, i18n

Motivation / Impact

Users can inspect the original provider error and malformed tool-call payload when an automatic retry occurs, without changing the default retry behavior. Diagnostics remain collapsed until explicitly opened.

Verification

• Passed: pnpm run fmt:rs
• Passed: pnpm --dir src/web-ui run test:run src/flow_chat/store/FlowChatStore.test.ts (57 tests)
• Passed: pnpm run type-check:web
• Passed: cargo test -p bitfun-events (16 tests)
• Passed: git diff HEAD~1..HEAD --check
• Not completed: cargo check -p bitfun-core exceeded the local 60-second command limit twice while checking bitfun-core; no compiler errors were emitted.

Reviewer Notes

• ModelRoundAttemptSuperseded is a high-priority realtime event; receipt updates UI state but does not force an immediate persistence write.
• The session field is optional/defaulted, so existing stored sessions restore without migration.
• Raw diagnostics can be long; the UI keeps them behind a warning-icon disclosure and supports copying individual values.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, including the incomplete core check.
• User-facing strings and locales are updated for en-US, zh-CN, and zh-TW.