chore: Update ZG airline enum from “Groznyy Avia” to “ZIPAIR”

[dzintt]

This PR updates the ZG airline enum value from "Groznyy Avia" to "ZIPAIR".

Why this change is needed

ZG is currently used by ZIPAIR, the Japanese low-cost carrier.
Groznyy Avia is a defunct airline, and keeping ZG = "Groznyy Avia" causes incorrect airline labeling in current usage.

This can lead to confusion for anyone using the library with modern flight data, where flights marketed or operated by ZIPAIR may be displayed under an outdated and unrelated airline name.

What this fixes

• Aligns the enum with current airline code usage
• Prevents incorrect mapping of ZIPAIR flights to Groznyy Avia

Notes

This appears to be a stale historical mapping rather than the correct present-day airline name for ZG.

Greptile Summary

This PR corrects a stale IATA airline code mapping by updating ZG from "Groznyy Avia" (a defunct Russian carrier) to "ZIPAIR" (the active Japanese low-cost carrier operated by JAL Group). The change is made consistently across both the raw data/airlines.csv source file and the derived fli/models/airline.py enum.

• The mapping is factually correct: IATA code ZG is officially assigned to ZIPAIR Tokyo (confirmed via Wikipedia, Kayak, Airhex, and avcodes.co.uk).
• Both files are updated in sync — the enum key ZG is unchanged and only the string value is updated.
• No logic, tests, or other dependent code is affected by this value change.

Confidence Score: 5/5

Safe to merge — a minimal, factually verified data correction with no logic changes.

Both changed files are updated consistently, the new value is confirmed correct by multiple authoritative IATA sources, and no code logic or API contracts are affected.

No files require special attention.

Important Files Changed

Filename	Overview
data/airlines.csv	Updates ZG airline entry from "Groznyy Avia" to "ZIPAIR" — factually correct per official IATA records
fli/models/airline.py	Updates Airline enum ZG value from "Groznyy Avia" to "ZIPAIR", keeping enum key unchanged and in sync with the CSV source

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["data/airlines.csv\nZG,ZIPAIR"] -->|"Enum generated from CSV"| B["Airline enum\nZG = 'ZIPAIR'"]
    B -->|"Parsed by"| C["fli/core/parsers.py"]
    C -->|"Used in"| D["SearchFlights / SearchDates\nairlines filter"]
    D -->|"Returns results with"| E["FlightResult\nwith correct airline label: ZIPAIR"]

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[punitarani]

nit: let's do "Zipair" or "Zipair Tokyo" instead