Add electric-has-data response header

[balegas]

Summary

• Adds a new electric-has-data response header ("true" / "false") to all successful shape responses, allowing operators to distinguish data-bearing from empty responses without parsing bodies
• Fixes the :now offset path to correctly set no_changes: true on the response
• Auto-exposes the new header via CORS by adding it to @electric_headers

Fixes #3850

Test plan

• Added refute response.no_changes assertion on data response test in api_test.exs
• Added assert response.no_changes assertion on "now" offset and long-poll timeout tests in api_test.exs
• Added electric-has-data header assertions in serve_shape_plug_test.exs for data responses ("true") and empty responses ("false")
• CORS test passes automatically since header is in @electric_headers
• All 70 tests pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.17%. Comparing base (b3ae567) to head (0bcd48f).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3851       +/-   ##
===========================================
+ Coverage   69.43%   87.17%   +17.74%     
===========================================
  Files           6       25       +19     
  Lines         386     2308     +1922     
  Branches       96      577      +481     
===========================================
+ Hits          268     2012     +1744     
- Misses        117      294      +177     
- Partials        1        2        +1     

Flag	Coverage Δ
packages/experimental	87.73% <ø> (ø)
packages/react-hooks	86.48% <ø> (?)
packages/start	82.83% <ø> (?)
packages/typescript-client	92.07% <ø> (?)
packages/y-electric	56.05% <ø> (ø)
typescript	87.17% <ø> (+17.74%)	⬆️
unit-tests	87.17% <ø> (+17.74%)	⬆️

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[balegas]

@KyleAMathews I want to add this header to be able to filter out live requests with no data in OTEL. You might have opinions about it.

I don't think the latency or the amount of these requests are useful for observability and removing them would remove a lot of noise from our dashboards