fix(agent): report MongoDB wire message lengths

[jiangjiang248]

This PR is for:

• Agent

Fixes MongoDB request and response lengths being reported as zero

Steps to reproduce the bug

1. Parse either existing MongoDB capture in agent/resources/test/flow_generator/mongo/.
2. Inspect the resulting MongoDBInfo: req_len and resp_len remain zero even though the MongoDB message header contains a nonzero length (for example, 230 bytes for the request and 285 bytes for the response in mongo-msg.pcap).

Changes to fix the bug

• Assign the header length to the MongoDBInfo returned by parse_payload, instead of the parser's internal self.info.
• Add a unit test showing that request and response lengths come from the wire header even when fewer bytes were captured.
• Update the two capture test expectations for all 32 parsed MongoDB messages.

Affected branches

• main

Checklist

• Added a unit test to verify the fix.
• rustfmt --edition 2021 --check agent/src/flow_generator/protocol_logs/sql/mongo.rs
• Checked the 32 updated capture expectations against the message lengths in both PCAP files.
• Run the Agent Cargo test suite in CI. A full repository checkout was unavailable in the local environment.

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