Add network/client span attributes and fix span status per OTEL semconv

[ChrisJBurns]

Summary

• Adds new additive span attributes for better observability:
  • network.protocol.version from incoming HTTP request protocol
  • mcp.backend.protocol.version from backend transport (SSE→"1.1", streamable-http→"2")
  • client.address and client.port from RemoteAddr
  • mcp.session.id from Mcp-Session-Id header
  • error.type on 5xx errors
• Fixes span status to follow OTEL HTTP server span semantic conventions:
  • 4xx responses → Unset (client errors, not server errors per OTEL spec)
  • 5xx responses → Error with error.type attribute
  • 2xx/3xx → Ok (unchanged)
• Separates frontend and backend protocol versions into distinct attributes to avoid silent overwrite when they differ (e.g., HTTP/2 client with SSE/HTTP1.1 backend)
• Extracts addNetworkAttributes helper to keep cyclomatic complexity within limits

Protocol version attributes:

Attribute	Source	Example
network.protocol.version	Incoming HTTP request	"2" (HTTP/2 client)
mcp.backend.protocol.version	Backend MCP transport	"1.1" (SSE backend)

Stacked on #3748 (attribute renames) and #3747 (feature flag).

Test plan

• New unit tests for httpProtocolVersion helper
• New unit tests for parseRemoteAddr helper
• New tests for client.address, client.port, mcp.session.id attributes
• Negative test for mcp.session.id when header absent
• Test verifying HTTP/2 client + SSE backend produces distinct protocol versions
• Updated span status tests: 4xx → Unset, 5xx → Error with error.type
• task lint passes
• task test passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.41%. Comparing base (e15caa0) to head (4aa65ab).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/telemetry/middleware.go	95.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3750      +/-   ##
==========================================
+ Coverage   66.39%   66.41%   +0.01%     
==========================================
  Files         427      427              
  Lines       41864    41894      +30     
==========================================
+ Hits        27797    27825      +28     
  Misses      11956    11956              
- Partials     2111     2113       +2     

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