[Feature] Add MCP Protocol API as Alternative Interface to E2B

[bcfre]

Ⅰ. Describe what this PR does

┌─────────────┐    ┌─────────────┐
│   E2B API   │    │   MCP API   │   <-- External Protocol Interfaces
└──────┬──────┘    └──────┬──────┘
       │                  │
       └────────┬─────────┘
                ▼
        ┌───────────────┐
        │ SandboxManager│              <-- Unified Management Layer
        └───────┬───────┘
                ▼
        ┌───────────────┐
        │  Sandbox CRs  │              <-- Kubernetes Resources
        └───────────────┘

Both E2B and MCP APIs share the same underlying sandbox infrastructure - they are simply different protocol adapters over the unified sandbox management layer.

Changes
1. MCP Server Integration (cmd/sandbox-manager/main.go)

• Add MCP server alongside existing E2B server
• Configure via environment variables: MCP_SERVER_ENABLED, MCP_SERVER_PORT, MCP_SANDBOX_TTL, MCP_SESSION_SYNC_PORT

2. Sandbox Event Callback (pkg/sandbox-manager/infra/)

• Add SandboxEventHandler interface to propagate sandbox lifecycle events to MCP session layer
• Enable MCP sessions to track their associated sandbox state

3. Session-Sandbox Binding (api/v1alpha1/sandboxset_types.go)

• Add AnnotationMCPSessionID to link MCP sessions with underlying sandboxes

4. Expose Shared Components (pkg/servers/e2b/core.go, pkg/sandbox-manager/core.go)

• Expose SandboxManager and KeyStorage for MCP server reuse
• Add ListPeers() for MCP cluster discovery

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

[codecov]

Codecov Report

❌ Patch coverage is 79.27171% with 148 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.99%. Comparing base (edaad73) to head (136236a).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/servers/mcp/sandbox.go	63.52%	29 Missing and 2 partials ⚠️
pkg/servers/mcp/handler.go	87.94%	22 Missing and 5 partials ⚠️
pkg/servers/mcp/server.go	69.31%	24 Missing and 3 partials ⚠️
pkg/sandbox-manager/infra/sandboxcr/infra.go	4.76%	18 Missing and 2 partials ⚠️
pkg/servers/mcp/session.go	77.27%	18 Missing and 2 partials ⚠️
pkg/sandbox-manager/core.go	0.00%	9 Missing ⚠️
pkg/servers/e2b/core.go	0.00%	4 Missing ⚠️
pkg/servers/mcp/peer.go	93.84%	2 Missing and 2 partials ⚠️
pkg/servers/mcp/middleware.go	93.75%	1 Missing and 2 partials ⚠️
pkg/utils/sandbox-manager/proxyutils/default.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   74.65%   74.99%   +0.34%     
==========================================
  Files         141      150       +9     
  Lines        9836    10546     +710     
==========================================
+ Hits         7343     7909     +566     
- Misses       2183     2310     +127     
- Partials      310      327      +17     

Flag	Coverage Δ
unittests	74.99% <79.27%> (+0.34%)	⬆️

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[AiRanthem]

The coverage of 59.87805% seems to be not high enough

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kruise-bot]

@bcfre: PR needs rebase.

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