Tidy the public API before 1.0

[loevgaard]

Fixes #27

Stacked on #51.

Problem

The Roave BC check will lock the public surface in at 1.0, and a few things were public but unused, undocumented, or both.

Change

Kept and documented

• ConversionsApiEventRaised::$context is the way to hand your own listeners data that must never reach Meta, such as the order the event was raised for. It now says so in the docblock and in README.md, and it gained the getContext(string $key, mixed $default = null) accessor it was missing next to hasContext().
• CommandInterface is genuinely useful for routing every command the bundle dispatches as a group. The docblock now shows the routing snippet, and the README mentions it.

Tightened

• ConversionsApiEventRaised::$event and ::$context are readonly. The Event object stays mutable, which is what enrichment listeners need, but the properties can no longer be swapped.

Removed

• The setono_meta_conversions_api.client_side.enabled and .server_side.enabled container parameters. No service read them once the subscribers became conditional, and whether a side is on is already visible from whether its services exist.

Tests

A test for getContext() including the default, and the extension test drops its assertions on the two removed parameters.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (fix/29-cookie-domain@e7e75f9). Learn more about missing BASE report.

Additional details and impacted files

@@                   Coverage Diff                   @@
##             fix/29-cookie-domain      #52   +/-   ##
=======================================================
  Coverage                        ?   90.60%           
  Complexity                      ?      167           
=======================================================
  Files                           ?       34           
  Lines                           ?      543           
  Branches                        ?        0           
=======================================================
  Hits                            ?      492           
  Misses                          ?       51           
  Partials                        ?        0           

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