feat: add operationId, tag rules, and envelope-aware response splitting

[Arkptz]

Summary

Adds three backwards-compatible features to generate:

1. operationId generation (--operation-id-strategy)

Derives stable camelCase operationIds from HTTP method + path template so openapi-generator-cli produces readable Rust method names:

GET /api/v1/users           → listUsers
GET /api/v1/users/{id}      → getUser
POST /api/v1/users          → createUser
GET /api/v1/contract/fair_price/{symbol} → getFairPrice
POST /api/v1/private/order/place         → placeOrder

Flags: --operation-id-strategy {none,path,template}, --operation-id-overrides <yaml>, --operation-id-template <tmpl>

2. Regex-based tag rules (--tag-rules)

Assigns one tag per operation via a YAML rules file (first-match-wins). Tags are used by openapi-generator to split operations into separate source files:

rules:
  - match: "^/api/v1/contract/"
    tag: Contract
  - match: "^/api/v1/private/"
    tag: Private
default: Market

Flags: --tag-strategy {legacy,none,path-segment,rules}, --tag-rules <yaml>, --tag-segment-index <N>

3. Envelope-aware response splitting (--envelope-discriminator)

MEXC and similar exchange APIs always return HTTP 200 with a success boolean discriminator. This flag splits captured bodies into per-operation {OperationId}Success schemas and a shared components/schemas/ApiError, connected via oneOf + discriminator annotation:

mitm2openapi generate ... \
  --operation-id-strategy path \
  --tag-rules tag-rules.yaml \
  --envelope-discriminator success

Flags: --envelope-discriminator <field>, --envelope-error-shape <yaml>, --envelope-success-component-suffix <string>

4. Deterministic output

Sorts all paths and component schemas alphabetically before serialization so diffs are byte-stable across runs.

Backwards Compatibility

All new flags default to the v0.6.0 behaviour — snapshot test tests/snapshot_compat.rs enforces byte-identical output when no new flags are passed.

Tests

• 306 unit tests (including 9 tag_rules + 17 operation_id + 8 envelope tests)
• 1 snapshot test (byte-identical backwards-compat guard)
• 2 end-to-end integration tests (all 3 features combined, and no-flags backward compat)
• cargo clippy --all-targets -- -D warnings clean

New files

• src/tag_rules.rs — regex-based tag matching module
• src/operation_id.rs — camelCase operationId derivation + collision resolution
• src/envelope.rs — discriminator-based response splitting