Add unit tests for the Cohere buildCohereCountTokensText helper

[deepintai]

Context

buildCohereCountTokensText flattens Responses messages into a single plain-text payload for Cohere tokenization. It handles several message shapes (plain text, content blocks, reasoning summaries) but has no dedicated unit tests, and there is no count_tokens_test.go next to the implementation yet.

What to do

Create deepintshield_server/core/providers/cohere/count_tokens_test.go (using the same style as rerank_test.go) with cases for:

1. A single text message.
2. Multiple sequential messages.
3. An empty message slice (must not panic; returns empty string).
4. Messages whose content is a list of content blocks containing text.
5. Messages carrying reasoning summaries.
6. Mixed content (text + blocks + reasoning).
7. Messages with nil fields.
8. Messages with empty/whitespace strings (verify trimming behavior).

Acceptance criteria

• New file deepintshield_server/core/providers/cohere/count_tokens_test.go.
• All 8 cases present and passing via go test ./core/providers/cohere/....
• Uses standard Go testing + testify assertions, matching rerank_test.go.
• Each case asserts the exact expected flattened string (or that it is empty where appropriate).
• No production-code changes required.

File pointers

• Function under test: deepintshield_server/core/providers/cohere/count_tokens.go:89 (buildCohereCountTokensText)
• Related schemas: deepintshield_server/core/providers/cohere/count_tokens.go (request/response types near the top)
• Test-style reference: deepintshield_server/core/providers/cohere/rerank_test.go