Typed column metadata reads via RetrieveMetadataChanges (one request, no @odata.type cast round-trip)

Summary

Reading type-specific column properties over /EntityDefinitions is a two-step dance: first read the generic attribute to get AttributeType/@odata.type, then re-issue a second GET with a derived-type URL cast (/Attributes/Microsoft.Dynamics.CRM.StringAttributeMetadata) to project the typed fields. The SDK does exactly this today (e.g. _bulk_fetch_picklists, extra_select="@odata.type,AttributeType").

Context: what the .NET modern client gives for free

ServiceClient gets fully-typed, polymorphic AttributeMetadata derived objects back in one RetrieveEntity/RetrieveAllEntities call and the caller just downcasts (attr is StringAttributeMetadata s). No second request, no discriminator juggling.

Proposed change

Add a typed read path backed by the Web API RetrieveMetadataChanges function -- the closest Web-API analog to the .NET single-request typed retrieve. It returns derived typed attributes in one call and supports property projection (via the query's AttributeQuery.Properties), so callers get small, typed payloads without the cast round-trip.

cols = client.tables.list_columns("account", typed=True)   # typed values, one request
col  = client.tables.get_column("account", "emailaddress1", typed=True)

Why this does not need a platform change

Payload size is controlled by the query's property projection, so the platform "omit null fields" ask is not a blocker -- we simply project only the properties we need. (Note: the .NET client itself uses plain Retrieve* + a client cache rather than RetrieveMetadataChanges; for a Web-API client wanting small typed payloads, RetrieveMetadataChanges with projection is the better fit. Design discussion welcome.)

Touchpoints

• data/_odata.py -- new _retrieve_metadata_changes(...) issuing GET RetrieveMetadataChanges(Query=@p1)?@p1={...} with an EntityQueryExpression + AttributeQuery.Properties projection.
• operations/tables.py + async twin -- typed=True on list_columns / get_column (keep the existing cast path as fallback).
• Unit tests: request shape (projection, criteria) + typed mapping.
• README + CHANGELOG.

Acceptance

• list_columns(typed=True) returns typed values (e.g. MaxLength on string/memo, MinValue/MaxValue on int) in a single request, no @odata.type URL cast.
• Property projection limits the payload to requested properties.

Related

• #194 -- create-time column constraints (in progress).
• microsoft/Dataverse-skills#119 -- documents the current two-step typed-read workaround.

从 data/_odata.py 开始，检查现有的 metadata cast path，然后跟踪 operations/tables.py 中的 list_columns 和 get_column 及其 async twin。为 RetrieveMetadataChanges 的 request shape、projection、criteria 和 typed mapping 添加 unit coverage，并更新 README 和 CHANGELOG。完成的标准是：typed reads 能在一次 request 中返回 MaxLength 或 MinValue/MaxValue 等属性，同时保留现有的 cast path 作为 fallback。