centralize role resolution across all MCP tools

[anushakolan]

Related PR: #3737 — MSRC 31000000666371: MCP describe_entities info-disclosure fix + single-role alignment

Proposed fix

Add a single choke point on McpAuthorizationHelper that every MCP tool calls to obtain the caller's role:

public static bool TryResolveValidatedRole(
    HttpContext httpContext,
    IAuthorizationResolver authResolver,
    out string? role);

Behavior:

• Delegates validation to IAuthorizationResolver.IsValidRoleContext (exactly-one non-empty header value + HttpContext.User.IsInRole(header)).
• Returns the validated X-MS-API-ROLE header value verbatim as the single role for the request.
• Is the only place any MCP code reads AuthorizationResolver.CLIENT_ROLE_HEADER.

Then refactor every MCP tool to call it: DescribeEntitiesTool, AggregateRecordsTool, CreateRecordTool, DeleteRecordTool, ExecuteEntityTool, ReadRecordsTool, UpdateRecordTool, DynamicCustomTool.

Design

• Single-role model. X-MS-API-ROLE is one atomic role. No splitting, no unioning. Matches REST, GraphQL, and DAB's existing ClientRoleHeaderAuthorizationMiddleware.
• Resolver-owned inheritance. Per-entity authorization goes through IAuthorizationResolver.AreRoleAndOperationDefinedForEntity / GetAllowedExposedColumns, so anonymous → authenticated → named inheritance and wildcard All expansion are consistent with REST/GraphQL.
• One header read. AuthorizationResolver.CLIENT_ROLE_HEADER appears in exactly one MCP file after this change.

Acceptance

• grep AuthorizationResolver.CLIENT_ROLE_HEADER src/Azure.DataApiBuilder.Mcp/** returns one match, in McpAuthorizationHelper.
• No .Split(',') on the role header anywhere in the MCP project.
• All MCP tools use TryResolveValidatedRole; existing tests still pass.

Non-goals

• No resolver behavior changes.
• No config schema changes.
• No new live-database tests.

Reference

See PR #3737 for the single-role model, the MSRC fix in DescribeEntitiesTool, and the McpAuthorizationHelper.TryResolveAuthorizedRole alignment this issue builds on.