web UI: fix context notice using accumulated inputTokens instead of prompt snapshot

The context-usage banner in the web UI fell back to inputTokens when totalTokens was missing. inputTokens is accumulated across all API calls in a run (tool-use loops, compaction retries), so it overstates actual context window utilization -- e.g. showing "100% context used 757.3k / 200k" when the real prompt snapshot is only 46k/200k (23%).

Drop the inputTokens fallback so the banner only fires when a genuine prompt snapshot (totalTokens) is available.

Summary

• Problem: The web UI context-usage banner fell back to inputTokens (accumulated across all API calls in a run) when totalTokens (prompt snapshot) was missing, showing false alarms like "100% context used 757.3k / 200k".
• Why it matters: Users see a scary red "context full" warning when the actual context window is only 23% used, creating confusion and unnecessary session resets.
• What changed: Removed the inputTokens fallback in renderContextNotice so the banner only fires when a genuine totalTokens prompt snapshot is available. Added a test covering the missing-totalTokens edge case.
• What did NOT change (scope boundary): The status card text (Context: 46k/200k (23%)) was already correct and is untouched. Token accumulation, session storage, and per-message context % in grouped-render are unchanged.

Change Type (select all)

• Bug fix
• Feature
• Refactor required for the fix
• Docs
• Security hardening
• Chore/infra

Scope (select all touched areas)

• Gateway / orchestration
• Skills / tool execution
• Auth / tokens
• Memory / storage
• Integrations
• API / contracts
• UI / DX
• CI/CD / infra

Linked Issue/PR

• Related #(none identified)

User-visible / Behavior Changes

• The amber/red "X% context used" banner in the web UI no longer appears when totalTokens is unavailable, instead of falsely reporting accumulated inputTokens as context utilization.
• When totalTokens is present and genuinely high (>=85% of context window), the banner continues to display correctly.

Security Impact (required)

• New permissions/capabilities? No
• Secrets/tokens handling changed? No
• New/changed network calls? No
• Command/tool execution surface changed? No
• Data access scope changed? No

Repro + Verification

Environment

• OS: macOS
• Runtime/container: Bun / Node 22+
• Model/provider: minimax-portal/MiniMax-M2.5 (any provider reproduces)
• Integration/channel: Web UI (control-ui)
• Relevant config: default context window 200k

Steps

1. Start a session with a model that has a 200k context window.
2. Run several tool-use loops so inputTokens accumulates well past 200k (e.g. 757k).
3. Observe the context notice banner at the bottom of the chat view.

Expected

• Banner hidden (or showing accurate context % based on prompt snapshot).

Actual (before fix)

• Banner shows "100% context used 757.3k / 200k" even though real context utilization is 23%.

Evidence

• Failing test/log before + passing after
  • Existing test "hides the context notice when only cumulative inputTokens exceed the limit" already encoded the correct expectation and passes with the fix.
  • New test "hides the context notice when totalTokens is missing even if inputTokens is high" covers the undefined-totalTokens edge case.
• Screenshot/recording
  • User-provided screenshot showing the mismatch between status card (23%) and banner (100%).

Human Verification (required)

• Verified scenarios: All 29 chat view tests pass (28 existing + 1 new). Format and lint clean on touched files.
• Edge cases checked: totalTokens undefined with high inputTokens; totalTokens present and above 85% threshold; totalTokens present but below threshold.
• What you did not verify: Live web UI end-to-end with a real session (test-only verification). Pre-existing src/tts/tts.ts format failure on main unrelated to this change.

Review Conversations

• I replied to or resolved every bot review conversation I addressed in this PR.
• I left unresolved only the conversations that still need reviewer or maintainer judgment.

Compatibility / Migration

• Backward compatible? Yes
• Config/env changes? No
• Migration needed? No

Failure Recovery (if this breaks)

• How to disable/revert this change quickly: Revert the one-line change in ui/src/ui/views/chat.ts line 258 to restore ?? session?.inputTokens.
• Files/config to restore: ui/src/ui/views/chat.ts
• Known bad symptoms reviewers should watch for: Context notice never appearing even when context is genuinely near capacity (would require totalTokens to be permanently missing for active sessions — unlikely given session-usage persistence).

Risks and Mitigations

• Risk: If totalTokens is frequently undefined for active sessions, users lose the context-full warning entirely.
  • Mitigation: totalTokens is populated by deriveSessionTotalTokens on every reply with lastCallUsage or promptTokens. It is only missing on cold/stale sessions where context utilization is unknown anyway — showing nothing is more correct than showing a false alarm.