fix: migrate legacy llm_compress_keep_recent config

[zouyonghe]

What to build

Migrate existing user configs from the legacy provider_settings.llm_compress_keep_recent key to provider_settings.llm_compress_keep_recent_ratio so upgrades do not silently lose the user's context compression preference.

Acceptance criteria

• Existing configs with llm_compress_keep_recent and no llm_compress_keep_recent_ratio receive a deterministic migrated ratio.
• The old key is removed only after the new value is populated.
• Existing configs that already have llm_compress_keep_recent_ratio are not overwritten.
• Tests cover legacy-only, both-keys-present, and invalid legacy value cases.

Blocked by

None - can start immediately