fix: back off ollama embedding truncation retries

[Helweg]

Summary

Improve the Ollama embedding fallback path so context-length failures keep backing off instead of relying on a single optimistic truncation retry.

Changes

• replace the one-shot Ollama truncation retry with progressive fallback candidates in src/embeddings/provider.ts
• add provider-level regression coverage for near-limit and repeated context-length retry scenarios in tests/custom-provider.test.ts
• add indexer-level regression coverage proving split Ollama chunks recover without leaving failed batches in tests/indexer-failed-batches.test.ts

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #79