[Story]: app/lightbridge-authz/src/mcp.rs is split by tool group

### Story Statement
As an engineer adding an MCP tool, I want `mcp.rs` split by tool group, so that adding one tool does not mean opening a 3,274-line file.

### Real Intent
Second-largest file in the tree and the one most likely to grow, since every new MCP tool lands in it.

### Background and Context
3,274 LoC holding the MCP server plus every tool handler.

### Source of truth (links)
- Measured 2026-08-26
- `app/lightbridge-authz/src/mcp.rs`

### Acceptance Criteria
- [ ] Tools grouped by domain (accounts, projects, api-keys, validation)
- [ ] Server construction separated from tool handlers
- [ ] MCP tool names and schemas byte-identical before and after
- [ ] `just it-servers` passes unchanged

### Out of Scope
Adding or changing any tool.

### Dependencies and Blockers
Lower priority than `lib.rs`; scheduled after R2.

### Assumptions
Team capacity is 6 engineers; estimates are ideal dev-days for one engineer.

### Implementation Notes
Tool-name or schema drift is a silent breaking change for every MCP client. Diff the generated tool list before and after as a hard gate.

### Test Expectations
Capture the tool list before and after and assert equality; run `just it-servers`.

### Verification evidence
- [ ] Automated test added and proven to fail before the fix
- [ ] `just all-checks` green
- [ ] Every new or changed `src` file is <= 200 LoC

### Human accountable owner
@stephane-segning

### AI Usage Declaration
Story drafted with AI assistance from a verified repo audit.
Structure and estimates drafted with AI from a verified repository audit (file
line counts, config keys, and dependency state were read from the actual tree,
not assumed). A human owns intent, scope, and the release commitment.

### Human verification completed
- [ ] I can explain this work without referring to the AI-generated text
- [ ] Acceptance criteria are testable
- [ ] Source of truth is a real link, not boilerplate

Governance: https://adorsys-gis.github.io/ai-governance/