Fix #707 structured content persistence and harden streaming error handling

[afurm]

What this does

Fixes #707 by preserving structured string-keyed content across the ActiveRecord persistence boundary instead of reloading it as RubyLLM::Content::Raw.

This PR also hardens streaming error handling so the test matrix stays green:

• adds a useful to_s / to_json representation for Content::Raw
• handles missing Faraday v2 env objects in generic and Bedrock streaming error paths
• avoids VertexAI auth initialization during stubbed streaming error specs
• updates the gemspec spec to respect FARADAY_VERSION
• adds regression coverage for the persistence and streaming error cases

Fixes #707.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: #___

PRs for new features or enhancements without a prior approved issue will be closed.

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
• For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
• All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

Validation run locally:

• bundle exec rubocop
• Rails appraisal matrix for rails-7.1, rails-7.2, rails-8.0, rails-8.1
• The same appraisal matrix again with FARADAY_VERSION=1.10.3
• Local gitleaks scan against .gitleaks.toml

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes