chore: delete stale versioned clients when regenerating openapi/asyncapi codegen

[ayushsingh82]

Summary

generate-openapi-clients.ts and generate-asyncapi-clients.ts write Canton ledger-api output into core/ledger-client-types/src/generated-clients with the Canton version embedded in the filename (openapi-<version>.ts, -paths.ts, -provider-types.ts, asyncapi-<version>.ts). Every Canton version bump created a new versioned file without removing the old one. The repo currently has 6 Canton versions' worth of these files checked in (3.4.12, 3.5.1, 3.5.8, 3.5.10, 3.5.14) -- most no longer referenced by version-config.json at all.

Added cleanupStaleVersionedFiles() (scripts/src/lib/utils.ts), run at the start of both scripts, which deletes any openapi-*/asyncapi-* file whose embedded version isn't currently configured for any network (so running for one network doesn't delete the other network's valid file).

Scope note

This fixes the scripts going forward. It deliberately does not:

• delete the already-stale checked-in files (3.4.12, 3.5.1, 3.5.8), or
• touch core/ledger-client-types/src/index.ts, which still hardcodes openapi-3.4.12/openapi-3.5.1 as the '3.4'/'3.5' LedgerApiVersion union.

That '3.4'/'3.5' scheme turned out to be load-bearing business logic well beyond this package -- default client versions and version-parsing logic in core/ledger-client and core/asyncapi-client, plus hardcoded cantonVersion: '3.4' test fixtures in wallet-gateway/remote. Both currently supported Canton versions (3.5.10 mainnet, 3.5.14 devnet) are already in the same "3.5" family, so deciding which patch backs that key going forward -- or restructuring the versioning scheme entirely -- needs a maintainer's call, not something a codegen cleanup script should decide. Happy to take that on as a follow-up if a maintainer can weigh in on intent.

Fixes #1380

Test plan

• tsc -b clean in scripts/
• eslint / prettier --check clean on all 3 changed files
• Functionally tested cleanupStaleVersionedFiles against a fixture mirroring the repo's actual file set (all 6 checked-in Canton versions + an unrelated file): confirmed it removes exactly the 3.4.12/3.5.1/3.5.8 files (including .d.ts/.d.ts.map) and keeps 3.5.10/3.5.14 and the unrelated file
• Could not run the actual script:generate:openapi/script:generate:asyncapi end-to-end (needs network access to fetch specs from GitHub releases) -- the cleanup step itself is verified in isolation above

[ayushsingh82]

@alexmatson-da whenever you have a chance, would appreciate a look at this one.