feat(context): add reset_context/logging_context and category-aware host.json warnings

[yeongseon]

Closes #78

Summary

P1 follow-ups from the external review, split into two related concerns: invocation-context lifecycle and category-aware host.json conflict detection.

Context lifecycle

• Promote the previously private _reset_context_vars() helper to a public reset_context() API. Callers (custom middleware, test teardown, defensive wrappers) can now explicitly clear invocation context on reused workers.
• Add logging_context(context) — a context manager that calls inject_context() on enter and always calls reset_context() on exit, even when the wrapped block raises. This eliminates the most common footgun: stale invocation_id / cold_start leaking into the next invocation on a warm worker.
• with_context decorator now uses the new public reset_context() instead of its private duplicate, so there is a single source of truth for the reset semantics.
• README Quick Start now leads with with logging_context(context): and keeps inject_context / reset_context documented as the lower-level primitives for custom integrations.

host.json category-aware warnings

warn_host_json_level_conflict previously inspected only the default key under logging.logLevel. Real Azure Functions apps configure per-category levels (Function, Function.<name>, Host.Results, Host.Aggregator, …) and those silent suppressions were going undetected.

• Iterate every entry under logging.logLevel, resolve each level, and emit one warning per category that is more restrictive than the configured Python level.
• Warning text now distinguishes default from named categories (logLevel for category 'Function' is set to 'Warning' …).
• Non-dict logLevel blocks are tolerated (no warning, no crash).

Tests

• reset_context_clears_all, reset_context_idempotent
• logging_context_sets_then_resets, logging_context_resets_when_body_raises
• warns_per_category, no_warning_when_all_permissive, no_warning_when_log_level_block_is_not_a_dict
• does_not_install_duplicate_context_filters (idempotent ContextFilter install)
• only_website_instance_id_is_false (Azure-env detection edge case)
• test_public_api_exports and test_integration.test_public_api_exports updated for the two new exports.

Validation

• make lint — clean
• make typecheck — clean (24 source files)
• make test — 142 passed, 3 skipped (e2e), coverage 97.05%
• make build — sdist + wheel produced

Compatibility

• No breaking change. inject_context and with_context keep their existing semantics.
• Public surface gains two symbols: reset_context, logging_context. Both are typed and exported via __all__.
• No new runtime dependencies. Python 3.10+ preserved.