Fold OptimizedTagMap into a final class TagMap (#11967)

Fold OptimizedTagMap into a final class TagMap (drop the interface)

TagMap was an interface with a single implementation, OptimizedTagMap. The
split was vestigial scaffolding from when a second (HashMap-backed) impl
existed; with one impl it is false generalization. Collapse them into one
`public final class TagMap`:

- The interface's abstract method declarations are removed; OptimizedTagMap's
  bodies become TagMap's methods.
- Nested types that were implicitly `public static` in the interface
  (EntryChange, EntryRemoval, EntryReader, Entry, Ledger) are now written out
  explicitly as `public static`.
- Static factories (create/fromMap/ledger/...) and the EMPTY constant become
  explicit `public static` members; the EmptyHolder lazy-init note is updated
  now that there is no interface<->impl class-init cycle.
- putAll(TagMap) loses its `instanceof` dispatch (always true once there is one
  class) and calls the fast path directly.

No behavior change; motivation is code simplicity, not performance (a single
final class is monomorphic by construction, but CHA already devirtualized the
sole impl). Public API is preserved, so callers are unchanged; the 3 tests that
referenced OptimizedTagMap now reference TagMap.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Drop EmptyHolder, make factories final, simplify test-helper instanceof

Post-fold tidy, all TagMap-scoped:

- Remove EmptyHolder: with one class there is no interface<->impl class-init
  cycle to break, and the private constructor reads no statics, so EMPTY is a
  direct `new TagMap(new Object[1], 0)` initializer.
- Static factories (create/fromMap/ledger/...) are now `public static final`
  (not expressible on the old interface).
- assertSize/assertNotEmpty/assertEmpty/checkIntegrity test helpers dropped
  their now-always-true `instanceof TagMap` guard + redundant cast.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Restore null-contract annotations lost when folding away the interface

The interface -> final-class fold removed the TagMap interface decls, which
carried the @Nullable/@nonnull param annotations from #11963. Re-home them
onto the now-concrete methods: @nonnull tag keys and strict-setter values,
@nullable on the set(EntryReader)/getAndSet(Entry) sinks (+ the getAndSet
contract javadoc). The null-tolerance behavior was already preserved by the
fold; this restores the self-describing contract on the write surface.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Mark fromMap/fromMapImmutable map param @nonnull

Per review: these factories NPE on a null map (map.size()/putAll), so the
input is non-null by contract.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: dougqh