fix(srvb): unblock read for SRVB + correct inverted binding_category docs

[lin2qwer1-cloud]

Two independent SRVB defects found while building a RAP OData V4 service through the MCP surface against a live S/4HANA 2021 (SAP_BASIS 7.56) system. Both are small; the first one is what let the second one bite.

1. read SRVB is advertised but unreachable

GetSource's own tool description already lists SRVB (service binding) as a supported object_type, and pkg/adt implements it end to end (workflows_source.go case "SRVB" → Client.GetSRVB → JSON). But the MCP router's allow-list in routeSourceAction omits "SRVB", so the call is rejected:

No handler found for action="read" target="SRVB"
Supported read targets: CLAS, PROG, INTF, FUNC, FUGR, INCL, DDLS, BDEF, SRVD, ...

Consequence: a caller can create a service binding but cannot read back what it actually got. This is what made defect 2 silent.

Fix: add "SRVB" to the read allow-list — no new capability, it just reaches the existing implementation and matches the documented contract.

Verified live:

before: No handler found for action="read" target="SRVB"
after : {"name":"...","type":"SRVB/SVB","published":false,
         "bindingType":"ODATA","bindingVersion":"V4","serviceDefName":"..."}

2. binding_category is documented backwards

Per SAP's domain SRVB_BND_CATEGORY (verified on 7.56 via DD07L/DD07T):

value	meaning
0	UI ( User Interface )
1	A2X ( Application to X users ) — i.e. Web API

CreateObjectOptions, buildCreateObjectBody and the CreateObject tool schema all state the opposite ("0" for Web API, "1" for UI). A caller who wants a UI binding and follows the documentation passes "1" and silently gets an A2X/Web API binding.

The emitted default ("0") was already correct in practice — it produces a UI binding — only the label was wrong. Docs/semantics only, no behavior change.

Also clarified binding_version: it silently defaults to V2, producing a binding that cannot drive a Fiori Elements V4 app. Callers needing V4 must pass it explicitly; the schema now says so. (This is exactly how a binding named ..._O4 ended up as V2 for us, undetected because of defect 1.)

Tests

Added pkg/adt/crud_srvb_test.go pinning the ADT payload:

• category: default → srvb:category="0", explicit UI 0, explicit A2X 1
• version: default → srvb:version="V2", explicit V2 / V4
• service definition bound and upper-cased, srvb:type="ODATA"

go vet clean. internal/mcp and pkg/adt suites pass.

Note: the Recording* tests in pkg/adt appear flaky on this machine independently of this change — they pass in isolation on both this branch and clean main, and across repeated full-suite runs different Recording* tests fail on unchanged code (2 of 3 runs fully green). Unrelated to this PR, flagging in case it is useful.