refactor(adk): export backend struct types for external access

[JonXSnow]

Export struct types to enable direct type references from external packages.

Main changes:

• Rename sandboxToolBackend to SandboxToolBackend in agentkit package
• Rename backend to Backend in local package
• Update NewSandboxToolBackend() return type from filesystem.Backend interface to concrete *SandboxToolBackend
• Update NewBackend() return type from filesystem.Backend interface to concrete *Backend
• Update all method receivers and test assertions to use the exported type names

Impact:

• External packages can now access the concrete backend types directly
• Enables type assertions and direct struct field access when needed
• Maintains backward compatibility since the types still implement filesystem.Backend interface

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

• adk/backend/local (Current: adk/backend/local/v0.1.1)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

• adk/backend/local (Current: adk/backend/local/v0.1.1)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

• adk/backend/local (Current: adk/backend/local/v0.1.1)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/local (Current: adk/backend/local/v0.1.1)

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/local (Current: adk/backend/local/v0.1.1)

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• adk/backend/local (Current: adk/backend/local/v0.1.1)

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

⚠️ Please create and push new version tags for these modules after merging this PR.

[github-actions]

Need to create a new tag

The following modules have changes and may need version updates:

• components/model/gemini (Current: components/model/gemini/v0.1.28)

• components/model/ark (Current: components/model/ark/v0.1.64)

• components/model/claude (Current: components/model/claude/v0.1.15)

• adk/backend/local (Current: adk/backend/local/v0.1.1)

• adk/backend/agentkit (Current: adk/backend/agentkit/v0.1.0)

⚠️ Please create and push new version tags for these modules after merging this PR.