feat(envelopes): add Bedrock Agent envelope

[j-d-ha]

Description

Add support for Bedrock Agent events using the Bedrock Agent envelope.

Tasks

• Create package: AwsLambda.Host.Envelopes.BedrockAgent
• Implement BedrockAgentEnvelope<T> class
• Implement IRequestEnvelope interface
• Handle inputText extraction from Bedrock Agent invocations
• Create comprehensive README.md
• Add unit tests in tests/AwsLambda.Host.Envelopes.UnitTests/
• Update solution file
• Commit with conventional format

Acceptance Criteria

• inputText extraction working correctly
• Full unit test coverage
• README includes examples and AOT guidance
• Package builds for all target frameworks