Improve vague error messages in channel initialization and tool setup
to include specific config key paths and remediation steps, matching
the quality standard set by proxy validation errors.
Changes:
- telegram.rs: Include [channels.telegram] section path and required
fields (bot_token, allowed_users) in missing-config error; add
onboard hint; specify channels.telegram.allowed_users in pairing
message; improve parse error context
- whatsapp.rs: Specify channels.whatsapp.allowed_numbers key path
in unauthorized-number warning
- linq.rs: Specify channels.linq.allowed_senders key path in
unauthorized-sender warning; add onboard hint
- web_search_tool.rs: Include tools.web_search.provider config path
and valid values in unknown-provider error
Addresses API surface audit §8.2 (config context in error messages).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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