Restore default GNSS band and frequency mappings

[JianquanLu]

Summary

• use the existing per-system default band mappings when <band> is omitted
• keep the corresponding default frequency mappings available
• terminate the QZSS band and frequency switch cases explicitly

Problem

t_gsetgnss::_band() throws immediately when no band list is configured. This makes the default mappings directly below the exception unreachable, even though defaults are already defined for GPS, GLONASS, Galileo, BDS, and QZSS.

As a result, configurations that rely on the default behavior fail during initialization with:

You need set your band for ... in xml file

Fix

Remove the unconditional exception so the existing constellation-specific defaults are used, and add the missing break statements to the QZSS cases in both _band() and _freq().

Verification

Tested from commit 3b4f383a2724c1b0c4748cb0b0cb80e0bc430740 on Windows with MSVC Release:

• full Release build succeeds
• a one-epoch PPP run with all constellation <band>/<freq> settings omitted completes successfully (exit 0)
• the same run with explicit GPS band/frequency settings remains successful (exit 0)
• git diff --check passes

No observation data, generated products, binaries, or local paths are included in this PR.

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