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

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.10%. Comparing base (4942d6c) to head (a444caf).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/ruby_llm/content.rb	71.42%	2 Missing ⚠️
lib/ruby_llm/providers/bedrock/streaming.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
+ Coverage   87.05%   87.10%   +0.04%     
==========================================
  Files         119      119              
  Lines        5594     5615      +21     
  Branches     1407     1416       +9     
==========================================
+ Hits         4870     4891      +21     
  Misses        724      724              

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