Refactor providers and harden tool/response handling

[luckenco]

Summary

• refactor duplicated provider/build logic across core and provider modules
• harden tagged response deserialization and restore completion loop parallel control behavior
• simplify macro helpers and shared naming utilities
• clean up minor issues (error typo, docs/provider listing, clone usage)

Review fixes

• document why FunctionToolCall.r#type needs a serde default (tagged enum consumes the field during deserialization, but it's required when serializing back as an API input item)
• replace misleading deserialization test with two focused tests: variant selection and serialize round-trip
• inline unnecessary messages_clone variable in generate_completion
• document partial-failure semantics on process_parallel_function_calls

Validation

• cargo check --all --quiet
• cargo check --all --tests --quiet
• cargo clippy --all-targets --all-features --quiet
• cargo test --all --tests --examples --quiet
• cargo test -p rsai-macros --tests --quiet

Notes

• Full cargo test --all currently fails due macro doctest linker configuration (-liconv); targeted unit/integration/examples suites above pass.