node: add tests for httpService lifecycle and all servers

[VanshSahay]

Picks up where #1211 left off (which was closed for inactivity/incomplete coverage).

The gap has been sitting open since the regression caught in #1179 node/http.go wires up several servers but there were no tests verifying any of them actually boot and serve correctly. This adds that coverage. Rather than testing RPC methods (that's already handled in the rpc package), the tests focus on what this layer actually owns: can each server start, respond to at least one request, and shut down cleanly. Took inspiration from the old node/http/server_test.go that used to exist before the refactor.

• httpService starts and shuts down gracefully on context cancellation
• HTTP JSON-RPC server responds to valid requests and handles bad content-type without crashing (the class of bug from Fix grpc server #1179)
• WebSocket server completes the upgrade handshake (101 Switching Protocols)
• Metrics server serves /metrics with the right content-type
• pprof server serves /debug/pprof/
• Graceful shutdown under concurrent load

Each test gets ephemeral OS-assigned ports so nothing conflicts when run in parallel.
Closes #1180

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.31%. Comparing base (5e12792) to head (b5d56a7).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
node/http.go	80.00%	2 Missing ⚠️
node/node.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3487      +/-   ##
==========================================
- Coverage   75.40%   74.31%   -1.10%     
==========================================
  Files         385      385              
  Lines       34908    34976      +68     
==========================================
- Hits        26323    25993     -330     
- Misses       6734     6789      +55     
- Partials     1851     2194     +343     

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