fix(plugins): allow eager platform tool registration

[josh-hobbs-dawnleap]

Summary

• adds manifest-level eager_load: true support for bundled platform plugins
• marks the bundled A2A platform plugin eager so its outbound a2a_* tools register in normal chat sessions
• keeps ordinary platform plugins deferred by default
• adds regression coverage proving eager platform tools register while ordinary platforms remain lazy
• adds integration coverage loading the real bundled A2A plugin and asserting all five client tools register under the a2a toolset

Why

A2A is both an inbound gateway platform and an outbound agent-to-agent tool provider. Deferring the whole platform plugin means enabled A2A client tools can be missing from normal sessions until inbound platform materialization happens.

Test plan

• uv run --with pytest pytest tests/hermes_cli/test_plugins.py -q -o addopts=
• uv run --with pytest pytest tests/plugins/test_a2a_plugin.py -q -o addopts=
• git diff --check

[teknium1]

Closing in favor of PR #86660 (#86660), which fixed the same deferred-platform client-tool registration via the provides_tools mechanism the #78050 thread converged on (#78842 by @thelonewander3r, submitted a day earlier). Thanks for the fix — sorry it was a duplicate swarm.