fix(core): attach input to ExecutionContext for middleware and interceptors

[xiechimon]

Description

Tool arguments were never attached to ExecutionContext, so middleware and
interceptors could not inspect the actual input (context.input was always
undefined). This contradicts the SDK reference and Interceptor guide, both
of which show context.input as the canonical way to read tool arguments
in cross-cutting logic.

Changes Made

• Add optional input field to the ExecutionContext interface (types.ts)
• Populate context.input with the sanitised tool arguments at both call
  sites in server.ts (synchronous path and runTaskAsync)
• _meta is excluded from input as it already lives in context.metadata

Type of Change

• fix: Bug fix

Related Issues

Closes #328

Testing

• npm test — 708 tests pass, 60 suites green
• Added test: verifies context.input is populated with tool args
  and _meta is correctly excluded

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