feat: add Portal facade repoint client

[mikemcdougall]

Problem Summary

The server Portal facade and seed/bootstrap contract now exist, but honua-sdk has no client for repointing an ArcGIS-style Portal workflow to Honua /sharing/rest. Consumers currently need ad hoc HTTP for token issuance, Portal search, item lookup, and item-to-FeatureServer resolution.

Why It Matters Now

honua-server#2614 is closing the cross-SDK interop tail of the Portal facade. A Python path lets migration and automation code authenticate, discover Portal items, and open the existing GeoServices FeatureServer surface without duplicating wire semantics.

Acceptance Criteria

• Add async and generated synchronous Portal facade clients under honua_sdk.
• Accept either a Honua root URL or /sharing/rest URL and normalize it safely.
• Support generate_token, preserving the server contract that expires is Unix milliseconds.
• Support info, portals/self, paged search, and item lookup using Esri error envelopes.
• Resolve a Feature Service item URL to the existing GeoServices client path and carry the Portal token into subsequent requests.
• Add strict httpx mock tests for token, paging, errors, URL resolution, and credential propagation.
• Run scripts/gen_sync.py and keep generated sync/async surfaces aligned.
• Add an opt-in live proof against honua-server tests/seed/portal-compat.yaml, or cross-link the server compatibility lane that supplies it.
• Update public API compatibility evidence and docs.

Affected Repositories

• honua-sdk-python (implementation owner)
• honua-server (seed/contract and release-proof interop)

Gate-Tier Impact

• SDK lint/type/test/compatibility gates
• Server SDK compatibility evidence after release

Release/Deploy Impact

Requires a versioned honua-sdk release before the server compatibility manifest can claim the Python Portal surface. No server deployment change.

Non-Goals

• Reimplementing Portal server behavior in the SDK.
• UI, map rendering, interactive OAuth browser UX, or token refresh orchestration.
• Changes to the proprietary honua-gp or deprecated honua-arcpy packages.

Contract and Coordination

• Server follow-up: Portal repoint follow-ups: sdk-server-compatibility CI coverage + dotnet/python clients (#1373) honua-server#2614
• Cross-repo parent: Client repoint: SDK + @honua/sdk-esri-compat alignment with Portal facade + server seed/contract (#1240) honua-server#1373
• Server contract: docs/gis/PORTAL_FACADE_SEED_CONTRACT.md
• JS reference path: feat(esri-compat): Portal facade repoint client (#1373) honua-sdk-js#383