centralize role resolution across all MCP tools

**Related PR:** [#3737 — MSRC 31000000666371: MCP `describe_entities` info-disclosure fix + single-role alignment](https://github.com/Azure/data-api-builder/pull/3737)

## Proposed fix

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

```csharp
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](https://github.com/Azure/data-api-builder/pull/3737) for the single-role model, the MSRC fix in `DescribeEntitiesTool`, and the `McpAuthorizationHelper.TryResolveAuthorizedRole` alignment this issue builds on.

Lesen Sie McpAuthorizationHelper und die acht benannten MCP-Tool-Einstiegspunkte und vergleichen Sie anschließend die vorhandenen Alignment- und Sicherheitsänderungen in PR #3737. Verifizieren Sie, dass der Role-Header nur in McpAuthorizationHelper gelesen wird, kein MCP-Code ihn aufteilt, jedes Tool TryResolveValidatedRole verwendet und die vorhandenen Tests bestehen.

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