Teach the OpenAPI analyzer to inventory Rust module trees

[sdairs]

Parent: #317

Stack position: 1 of 7. This is the enabling change for every subsequent module move.

Problem

The analyzer currently parses client.rs, models.rs, and meta.rs as flat syn::File values and inventories top-level items only. Moving declarations behind private out-of-line modules such as mod services; pub use services::*; would make operations and models invisible to drift and response-policy checks.

Scope

• Make the analyzer follow inline and out-of-line Rust modules, including private modules containing public declarations.
• Inventory Client methods, structs, enums, aliases, VALUES constants, manual Default implementations, Serde defaults, and integer fields across the complete module tree.
• Update the analyzer executable and library APIs to accept the required source tree without duplicating Rust parsing in Python or tests.
• Update spec_coverage_test.rs, analyzer entrypoint coverage, and drift-script tests for the new input shape.
• Replace the raw models.rs SCIM substring count with analyzer-backed model inventory.
• Add a focused fixture using private nested modules and facade re-exports.

Acceptance

• The fixture proves nested models and impl Client methods are inventoried identically to flat equivalents.
• All analyzer policy inventories remain effective across modules.
• Running against the current flat source produces the same drift report and response tree as before.
• cargo test -p clickhouse-cloud-api -p clickhouse-openapi-analyzer passes.
• cargo clippy -p clickhouse-cloud-api -p clickhouse-openapi-analyzer --all-targets -- -D warnings passes.
• Python drift-script tests pass.

Out of scope

• Moving Cloud API models, methods, or conversions.
• Changing report semantics, the public API model policy, or drift exemptions.