Move HttpTransport hook registration to boot()

[mathetos]

Summary

Follow-up to #52 (review comment on #48): HttpTransport no longer registers
est_api_init in its constructor.

• Add boot() to McpTransportInterface for WordPress hook registration after construction
• McpTransportFactory calls boot() immediately after instantiating each transport
• HttpTransport::boot() keeps the existing rest_api_init priority (16) behavior
• Update custom transport docs and add a small unit test for the boot lifecycle

Motivation

Constructors should set up dependencies only; hook registration is explicit and easier to test in isolation. Happy to adjust naming or scope if maintainers prefer a lighter-touch approach (e.g. factory-only hook without an interface change).

Closes #52

Test plan

• CI unit/integration suite passes
• HttpTransportBootTest confirms the constructor does not register hooks and boot() registers rest_api_init at priority 16
• Smoke: MCP REST route still resolves after plugin activation (rest_url( 'mcp/mcp-adapter-default-server' ))

[github-actions]

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Co-authored-by: mathetos <webdevmattcrom@git.wordpress.org>
Co-authored-by: galatanovidiu <ovidiu-galatan@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.86%. Comparing base (90252e7) to head (08a2b4d).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #222   +/-   ##
=========================================
  Coverage     87.86%   87.86%           
- Complexity     1245     1246    +1     
=========================================
  Files            53       53           
  Lines          4037     4039    +2     
=========================================
+ Hits           3547     3549    +2     
  Misses          490      490           

Flag	Coverage Δ
unit	87.86% <100.00%> (+<0.01%)	⬆️

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