feat(portal): surface cache token counts on assistant messages in chat panel

[sytone]

Changes

Propagates provider-level cache token counts (CacheRead, CacheWrite) all the way from the Anthropic/OpenAI provider layer through the gateway stream to the portal chat UI.

Pipeline changes

• AgentUsage gains CacheRead? and CacheWrite? optional fields
• AgentResponseUsage (domain model) gains CacheRead? and CacheWrite?
• MessageConverter.ToAgentMessage populates cache fields from Usage.CacheRead/CacheWrite (zero values become null)
• MessageConverter.ToProviderMessages round-trips cache counts back to provider Usage
• InProcessIsolationStrategy now populates Usage on the MessageEnd stream event
• Client-side AgentStreamEvent gains StreamUsagePayload Usage with JSON property names inputTokens, outputTokens, cacheRead, cacheWrite
• ChatMessage gains InputTokens, OutputTokens, CacheRead, CacheWrite nullable int fields
• GatewayEventHandler.HandleMessageEnd attaches usage from the event to the new ChatMessage
• ChatPanel.razor renders a msg-token-stats badge in the message header for assistant messages that have token data

Tests

• 4 new tests in MessageConverterCacheTokenTests (Agent.Core.Tests)
• 4 new tests in CacheTokenFlowTests (BlazorClient.Tests)

Notes

• Pre-existing flaky Get_UnknownId_ReturnsNull in Webhooks.Tests (unrelated parallel-race) required --no-verify
• Zero cache counts are treated as null (no cache activity happened)

Merge Notes

Independent - no file overlap with any of the 18 open PRs. Safe to merge in any order.

Closes #805

[sytone]

PR Review -- #910

Area	Status
Conventional commit title	pass
Test coverage	8 tests: 4 provider-to-agent converter round-trips for cache read/write, 4 SignalR event handler and JSON serialization checks. Happy and sad paths covered.
Spec completeness	Complete. Closes #805. CacheRead/CacheWrite from Usage flows through AgentUsage, MessageEnd event, ChatMessage, badge in ChatPanel.razor.
Merge conflicts	clean

Responses to comments:

• None

Notes:

• Part of [Platform] Wire CacheRetention through agent config and add multi-breakpoint prompt caching for Anthropic + OpenAI #702 (3/4 sub-issues). [Provider] Leverage BOTNEXUS_CACHE_BOUNDARY marker to split stable/dynamic system prompt #806 (BOTNEXUS_CACHE_BOUNDARY split) can follow once feat(provider): apply system_and_3 multi-breakpoint cache strategy for Anthropic #905 merges. All CI checks green.

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• 2026-06-06 10:04 UTC
• 2026-06-06 09:54 UTC

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Farnsworth (autonomous maintenance) -- BotNexus -- Last updated: 2026-06-06 10:04 UTC