Unify dazpy protocol operations

[abrady]

Summary

• make sync and async dazpy clients own script registration, registered execution, request lifecycle, cancellation, SSE, and server-error mapping
• add ServerResponseError so protocol rejections remain typed at the dazpy boundary
• extend long-poll request timeouts beyond the server wait window
• keep all transport failures behind dazpy exceptions and preserve render-only cancellation semantics
• flush a valid final async SSE event even when the connection closes without a trailing blank line

Stacked dependency

• Depends on Expose structured observation for async jobs #37. This branch contains Expose structured observation for async jobs #37's structured-observation commits, including its Studio 4 threading fix, until that PR merges; the remaining diff is the unified-client layer.
• Both branches are based on current master / dazpy 2.9.2, including dazpy script-call batching + find_skeleton race fix #39's batching work.
• This is the final dazpy prerequisite for Make Daz MCP compact and dazpy-backed daz-mcp-server#9.
• nightly-70 is insufficient: it points at the dazpy script-call batching + find_skeleton race fix #39 merge and contains neither Expose structured observation for async jobs #37 nor Unify dazpy protocol operations #38. The earlier nightly-69 artifact produced 41 deterministic MCP failures because these APIs were absent.
• After Expose structured observation for async jobs #37 and this PR merge, MCP [Refactoring] Phase 7: Settings Management System #9 must be locked and tested against a nightly built from the resulting master commit, not against a version label alone.

Validation

• 749 passed, 30 subtests passed across tests/test_dazpy.py and tests/test_dazpy_aio.py
• built dazpy-2.9.2-py3-none-any.whl from this exact branch
• installed that wheel into an isolated checkout of Make Daz MCP compact and dazpy-backed daz-mcp-server#9: all 119 deterministic tests passed
• Python compileall and git diff --check passed
• the Studio 6 plugin layer compiled with INSTALL_TO_DAZ=OFF; no DLL was installed and Daz was not restarted

The validation above is offline/mocked; live acceptance remains deferred until the complete stack is published.