fix(a2a): treat a 0.3 historyLength of 0 as unlimited

[Superagentic-AI]

Why

The reference A2A Python SDK just fixed its 0.3 compat layer so that a v0.3 `historyLength` of `0` means unlimited on JSON-RPC and REST, matching its gRPC path (a2aproject/a2a-python#1311, fixed in #1312 on 2026-10-16). SuperOptiX's server applied 1.0 semantics (zero means no history) to every caller, so a 0.3 client sending `historyLength: 0` got an empty history from SuperOptiX but the full history from the SDK.

What

• `_history_length_param` and `_legacy_configuration` in `superoptix/protocols/a2a/server.py`: when the caller is on the 0.3 line (`A2A-Version: 0.3` or a 0.3 slash method), a zero historyLength is treated as unset.
• Applied to JSON-RPC `SendMessage` / `GetTask` and REST `/message:send` / `GET /tasks/{id}`.
• 1.0 behaviour is unchanged, so CORE-HIST-003 (zero returns no history) still holds.
• Four new tests in `tests/test_a2a_conformance.py::TestLegacyHistoryLengthZero` (0.3 JSON-RPC send and get, 0.3 REST get, and a 1.0 REST get guard).

Notes

• Small interop fix, no version bump, no release. `server.py` is identical on `release/2.0`, so this cherry-picks cleanly there.
• Locally (unlocked env) the new tests pass. `test_vary_names_the_negotiating_header` and a few `test_a2a_adapt` cases fail on unmodified main too in that env, so they are not from this change; CI on the locked env is the source of truth.