Fix valid_provider! raising ArgumentError instead of InvalidConfigError

[juniper-shopify]

Problem

Both Chat::Config#valid_provider! and Agent::Config#valid_provider!
raise ArgumentError when given an invalid provider name, despite their
doc comments explicitly promising InvalidConfigError:

# Note: this method will return the name of a valid provider
# or raise an `InvalidConfigError`.
def valid_provider!
  # ...
  raise ArgumentError, "'...' is not a valid provider..."
end

This means any caller writing rescue InvalidConfigError — following
the documented contract — will not catch invalid provider errors. The
exception escapes as an unhandled ArgumentError instead of flowing
through the ConfigError hierarchy like every other config validation.

Fix

Changed both methods to raise InvalidConfigError (which they inherit
from Cog::Config via the class hierarchy). This is consistent with:

• Chat::Config#valid_api_key! (line 124), which already raises
  InvalidConfigError for missing API keys
• Map::Config#valid_parallel! (line 89)
• Config#working_directory (lines 297–298)
• All Config#validate! overrides across the cog tree

Updated the corresponding tests in both chat/config_test.rb and
agent/config_test.rb to assert Cog::Config::InvalidConfigError
instead of ArgumentError.

[juniper-shopify]

• [DRAFT] Add workflow to validate and update internal architecture documentation #892
• Add LLM-generated detailed architecture documentation #868
• Fix valid_provider! raising ArgumentError instead of InvalidConfigError #964 👈 (View in Graphite)
• main

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