test: add systematic test coverage for 7 bug pattern groups (#852)
Add ~105 test cases across 7 test groups identified in issue #852:
TG1 - Provider resolution (27 tests): Factory resolution, alias mapping,
custom URLs, auth styles, credential wiring
TG2 - Config persistence (18 tests): Config defaults, TOML roundtrip,
agent/memory config, workspace dirs
TG3 - Channel routing (14 tests): ChannelMessage identity contracts,
SendMessage construction, Channel trait send/listen roundtrip
TG4 - Agent loop robustness (12 integration + 14 inline tests): Malformed
tool calls, failing tools, iteration limits, empty responses, unicode
TG5 - Memory restart (14 tests): Dedup on same key, restart persistence,
session scoping, recall, concurrent stores, categories
TG6 - Channel message splitting (8+8 inline tests): Code blocks at boundary,
long words, emoji, CJK chars, whitespace edge cases
TG7 - Provider schema (21 tests): ChatMessage/ToolCall/ChatResponse
serialization, tool_call_id preservation, auth style variants
Also fixes a bug in split_message_for_telegram() where byte-based indexing
could panic on multi-byte characters (emoji, CJK). Now uses char_indices()
consistent with the Discord split implementation.
Closes #852
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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