feat: OpenTelemetry instrumentation + Agent365 baggage support

[rido-min]

Summary

• Emits per-turn ActivitySource spans (turn / middleware / handler / auth.outbound / conversation_client) and Meter instruments (teams.activities.received, teams.turn.duration, teams.handler.errors, teams.middleware.duration, teams.outbound.calls, teams.outbound.errors) from the SDK pipeline. Each layer owns its own source: Core → Microsoft.Teams.Core, Apps → Microsoft.Teams.Apps.
• Adds two layer-specific BaggageBuilder classes for Agent365 export (same name in different namespaces, no inheritance). Core's reads from CoreActivity / ConversationAccount with a channelData.tenant.id JSON fallback; Apps's adds the TeamsConversationAccount-backed keys (user.id, user.email, microsoft.agent.user.email, gen_ai.agent.description) and uses typed TeamsChannelData for the tenant fallback.
• Promotes TenantId to a typed property on Core's ConversationAccount so the Core layer can populate microsoft.tenant.id without Apps-layer dependencies.
• Adds the design doc at core/docs/Observability-Design.md (layering rules, span/metric maps, crisp Agent365 cert definition with per-scope attribute tables, Channel/SubChannel mapping resolution) and a runnable sample at core/samples/ObservabilityBot/ that wires UseMicrosoftOpenTelemetry with OTLP export.
• Picks up one new package dep on Microsoft.Teams.Core: OpenTelemetry.Api 1.15.3 (lightweight contract package — needed for OpenTelemetry.Baggage.Current; transitive on every Microsoft.OpenTelemetry consumer).

Test plan

• dotnet build clean on net8.0 + net10.0 for Microsoft.Teams.Core, Microsoft.Teams.Apps, Microsoft.Teams.Apps.BotBuilder, and samples/ObservabilityBot.
• dotnet test core/test/Microsoft.Teams.Core.UnitTests — 111 passing on both TFMs (96 baseline + 15 new for telemetry / baggage).
• dotnet test core/test/Microsoft.Teams.Apps.UnitTests — 161 passing on both TFMs (152 baseline + 9 new for handler span / baggage).
• dotnet test core/test/Microsoft.Teams.Apps.BotBuilder.UnitTests — 41 passing (no regressions).
• Smoke-run samples/ObservabilityBot — Microsoft.OpenTelemetry distro 1.0.1 boots cleanly, log records flow through OTel with the full resource bag.
• End-to-end Agent365 export validation against a real tenant (deferred — no Agent365 credentials in this environment).

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