Add PECL package ecosystem support

[AgentGoose32]

Summary

• add a PECL ecosystem adapter backed by PECL's REST XML endpoints
• map package names, package metadata, release metadata, registry URLs, install commands, and status URLs
• seed pecl.php.net as a default package registry
• add model coverage for PECL package names, metadata, releases, URLs, install commands, and purl type

Refs #708

Validation

• ruby -c app/models/ecosystem/pecl.rb
• ruby -c db/seeds.rb
• ruby -c test/models/ecosystem/pecl_test.rb
• git diff --check

Full Rails test execution is blocked locally because this repo lockfile requires Bundler 4.0.10 while the system Ruby/Bundler cannot satisfy it.