Preserve asynchronous observability context

[anth-volk]

Fixes #31

Changes

• restore configured dispatch attributes when a remote asynchronous operation starts
• let locally selected attributes override values received from the dispatching process
• include active dispatch attributes in structured records without adding them to metric labels
• attach active dispatch attributes, including observability_id, to every nested span
• accept explicitly supplied safe scalar attributes in local logs and spans by default, while retaining optional strict local filtering
• restrict asynchronous transport to dispatch_attribute_keys and metric labels to metric_attribute_keys
• remove the numeric limit on accepted attributes while retaining prohibited-name, value-type, redaction, and string-length checks
• isolate nested and concurrent runtime context with context-variable tokens
• ignore malformed remote context without altering application execution
• document the separate local, asynchronous transport, and metric attribute policies

Release and consumers

The fixed changelog fragment causes the release workflow to publish policyengine-observability 3.0.1 after merge. PolicyEngine/policyengine-sim-api#686 and PolicyEngine/policyengine-api#3850 must then update their dependency constraints and lockfiles to require 3.0.1.

Validation

• 135 package tests passed.
• Coverage remained at or above the required 90% threshold.
• Ruff formatting and lint checks passed.
• Pyright passed.
• The Towncrier changelog check passed.