Trim the migration guide to genuine v1-to-v2 breaking changes

The guide had accumulated entries that are not v1-to-v2 migration
items: new v2 feature notes, "what did not change" reassurance
sections, spec-adoption commentary, deprecations of APIs that still
work, and internal fixes with nothing for a migrator to change. Remove
those, including the whole Deprecations group; the two deprecation
consequences a migrator does act on (warnings-as-errors test runs, and
client calls that die on the default 2026-era connection) live in the
testing section.

Correct code and claims that were wrong for v1.x or stale for current
2.x, and verify every remaining before/after example against the v1.x
and main trees. Two behavior changes that shipped in 2.1 without a
guide entry get one: exception messages from tool handlers no longer
reach the model unless raised as ToolError, and content-block return
annotations no longer produce structured output. Reframe the 2026-era
notes as behavior changes migrated code sees on v2's default connection,
fold closely related entries together, standardize on the documented
imports, regenerate the index and suggested-order blocks, and cut
history and over-explanation throughout.

Also drop two whats-new.md links whose target sections this removes.

No-Verification-Needed: documentation-only change

Author: maxisbey