fix(sync): shrink Gmail fetch page to avoid Composio 413

[senamakel]

Summary

Follow-up to #71. Live Gmail sync failed with HTTP 413 Payload Too Large from
Composio's execute endpoint.

GMAIL_FETCH_EMAILS is called with include_payload: true, so each page returns
full message bodies. A 50-message page overflows Composio's tool-response size
cap. Verified against a real mailbox:

max_results (include_payload: true)	Result
50	HTTP 413
25	HTTP 200
10 / 5	HTTP 200

Lower the Gmail pipeline's default page_size from 50 to 25 (which also
matches the default per-toolkit item budget). Callers needing more throughput can
still raise it via with_limits.

Testing

• cargo test --features sync --test composio_sync_mock — 12 passed.
• Live: COMPOSIO_TOOLKITS=gmail cargo run --example composio_harness --features sync
  → was 413, now PASS, 25 documents ingested.

https://claude.ai/code/session_01AtynpEKn3QZJKZYvL8uDNR

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📥 Commits

Reviewing files that changed from the base of the PR and between 02cfc2a and 3cb3616.

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• src/memory/sync/composio/gmail.rs

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