feat: Add AWS Bedrock LLM provider support with token auth

[smitb]

Summary

Adds AWS Bedrock as a supported LLM provider via the Converse API using
Bedrock API Key bearer token authentication. Credentials resolve from
bedrock_api_key in config or AWS_BEARER_TOKEN_BEDROCK env var, with
region defaulting to us-east-1.

Changes

• New bedrock.rs driver implementing the full LlmDriver trait (completions, streaming, tool use, token usage)
• bedrock registered as a named provider in create_driver()
• Bedrock added to the init wizard provider list, defaulting to anthropic.claude-sonnet-4-6

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[Jengro777]

Quick feedback from the community:
https://github.com/librefang/librefang