feat(channels): echo user messages from other channels to Telegram conversations

[sytone]

Closes #320

Changes

• Added FanOutUserMessageAsync to GatewayHost: after user message persistence, sends the user message to all other bindings in the conversation with isUserEcho metadata
• TelegramChannelAdapter detects user echo messages and formats as User Said: prefix
• Added EchoForeignUserMessages config flag to TelegramBotConfig (default true, opt-out)
• Non-Telegram adapters receive the echo but pass it through unchanged
• Updated 8 existing MultiChannelFanOutTests to account for user echo messages
• Created BotNexus.Extensions.Channels.Telegram.Tests project with 8 unit tests

Merge Notes

This PR touches GatewayHost.cs and BotNexus.slnx. GatewayHost.cs is not in any other open PR. BotNexus.slnx overlap with #1018 and #914 is in different sections (additive). Safe to merge in any order.

[sytone]

PR Review -- #1024

Area	Status
Conventional commit title	pass
Test coverage	8 new Telegram tests + 8 updated multi-channel fan-out tests covering user echo delivery, config opt-out, and message formatting
Spec completeness	complete -- all ACs from #320: foreign user echo, native suppression, agent pass-through, config toggle, unit tests
Merge conflicts	clean

Responses to comments:

• None

Notes:

• BotNexus.slnx has additive overlap with feat(tools): add platform debug tool for read-only session and runtime inspection #1018 and feat(provider): add GitHub Models inference provider with free-tier model catalog #914 (different project entries). GatewayHost.cs not in any other PR.

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• 2026-06-08 04:26 UTC

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Farnsworth (autonomous maintenance) -- BotNexus -- Last updated: 2026-06-08 04:26 UTC