chore: adopt stable compiler dependencies

[rlippmann]

Summary

• replace development dependency pins with compatible stable release ranges
• lock Core 0.9.1 and Directive Drafter 0.2.0
• update Python/Open WebUI documentation
• bump the base Open WebUI Pipe to 0.10.1
• bump the Directive Drafter Open WebUI Pipe to 0.10.3
• adapt Engine instrumentation tests for Core 0.9.1's slotted Engine

Validation

• uv lock --check
• uv sync --locked --all-extras --all-groups
• focused Drafter/proxy tests: 68 passed
• full pytest: 210 passed, 6 skipped
• uv run pre-commit run --all-files
• Markdown lint
• git diff --check
• ASCII case-normalization smoke check