fix: regenerate the stale avp_pb2 so import avp works with the protobuf 5.x runtime

[LuMarans30]

Problem

import avp fails with a VersionError whenever the installed protobuf is
older than the linked gencode:

$ python -m venv /tmp/repro && . /tmp/repro/bin/activate
$ pip install -e ".[all]"
$ python -c "import avp"
google.protobuf.runtime_version.VersionError: Detected incompatible
Protobuf Gencode/Runtime versions when loading avp.proto:
gencode 6.31.1 runtime 5.29.6.

Root cause

pyproject.toml declared only protobuf>=4.21, while the checked-in
src/avp/avp_pb2.py was compiled with gencode 6.31.1, whose runtime guard
rejects any protobuf older than 6.31.1. So every environment constrained
below 6.31.1 failed at import avp. The all extra hits it deterministically
because autogen-core requires protobuf~=5.29.3, so pip install "avp[all]"
can never supply the runtime the stale gencode demands.

While regenerating, I also noticed the checked-in file had drifted from the
schema. It was last generated on 2026-03-24 (658aca2), right before 6f7d04e
removed EMBEDDING in v0.5.0. Since it was never recompiled,
avp_pb2.PayloadType still defined EMBEDDING = 2.

Fix

• Recompiled src/avp/avp_pb2.py using protoc 5.29 (gencode 5.29.0).
• Raised the minimum protobuf requirement from >=4.21 to >=5.29.
• Pinned grpcio-tools to >=1.71.2,<1.72. grpcio-tools 1.72+ targets
  protobuf>=6.30 (1.84+ targets >=7.35.1), neither of which can coexist
  with autogen-core's pin (~=5.29.3). Left unpinned, the next make proto
  would reintroduce the mismatch and break installing dev alongside all.

proto/avp.proto is untouched. The only change in the generated code is
dropping the unused EMBEDDING value. The public PayloadType enum in
types.py never had it, so there are no public API changes.

Compatibility

• The protobuf floor moves from >=4.21 to >=5.29: environments pinned to
  4.21-5.28 must upgrade. protobuf 4.x is end-of-life, and a 5.29-generated
  module cannot load on those runtimes.
• No upper bound is needed: gencode 5.29 loads on newer runtimes too (verified
  on 7.36.1).
• Peers on released 0.5.x/0.6.x still carry EMBEDDING = 2; codec.py already
  rejects unknown payload_type values, so decoding behavior is unchanged.

Verification

• protobuf 5.29.6 and 7.36.1: import avp succeeds, avp_pb2.PayloadType
  contains only {HIDDEN_STATE, KV_CACHE}.
• make proto with grpcio-tools==1.71.2 reproduces the committed file exactly
  (git diff clean).
• tests/test_codec.py and tests/test_codec_v2.py pass (28 tests total).