Simplify variant dispatch and session by replacing per-method
boilerplate with generic handlers backed by reflect.
## Motivation and Context
The dispatcher and backend session had separate handler functions for
every MCP method (ListTools, ListResources, CallTool, Subscribe, etc.),
each following an identical pattern: extract params, inject metadata,
call inner handler, cast result. This meant ~250 lines of near-identical
code across dispatch.go and session.go, and every new MCP method
required adding another copy.
## How Has This Been Tested?
All existing unit and integration tests pass (go test -v ./...)
## Breaking Changes
None. All changes are internal to the variants package. Public API is
unchanged.
## Types of changes
Refactor (non-breaking change which fixes an issue)
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