feat: expose cache creation tokens by TTL to cost expressions

[lspataroG]

Description

Creating a cache entry is not priced at a single rate. Anthropic charges twice the
base input rate for an entry that lives an hour, against 1.25x for one that lives
five minutes. Responses already report which was used, in usage.cache_creation,
but only the combined cache_creation_input_tokens was read — so a
costExpression cannot tell the two apart and under-charges the longer-lived
writes. On cache-heavy clients those writes can be the largest single item on a
bill, so the gap is not a rounding difference.

This adds cache_creation_5m_input_tokens and cache_creation_1h_input_tokens
as CEL variables, populated from usage.cache_creation on both the streaming and
non-streaming Anthropic paths. They can be used instead of the combined figure:

cache_creation_5m_input_tokens * 275u + cache_creation_1h_input_tokens * 440u

Nothing changes for existing users: the combined field keeps its meaning, and the
two new variables are 0 when a backend does not report the breakdown.

Related Issues/PRs (if applicable)

None.

Special notes for reviewers (if applicable)

• IsZero() and Override() were extended to cover the new fields, for
  consistency with the existing ones.
• Where a response reports cache_creation with zero values, the fields are
  marked as set rather than absent — present-and-zero is a different statement
  from not reported. Two existing test fixtures include that block, so their
  expectations were updated.
• Verified against live Vertex AI responses for both TTLs, streaming and not.
  make precommit and go test ./internal/... pass.
• Written with AI assistance; I have reviewed and tested the change.

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[lspataroG]

@theagentrouter/agent-router-maintainers — could someone approve the workflow runs on this PR? Build and Test, Precommit Check and CodeQL are all sitting in action_required, so no CI has actually run yet.

For the record, the red Netlify check is expected, not a failure: the deploy status is literally "Deploy Preview canceled". The last push only touched Go files, so nothing under the site/ base directory changed since the preview built for the previous commit, and Netlify skips the build in that case. The docs change in this PR was already rendered in that earlier preview.

Locally make precommit and go test ./internal/... pass. Happy to rebase or split anything out if that helps review.