fix: add decompile() handlers for custom xorq relation types

[paddymul]

Summary

• Adds failing tests for decompile() has no handler for FlightUDXF (and other custom xorq relation types) #1702 — decompile() has no @translate.register() handlers for CachedNode, RemoteTable, FlightUDXF, or Read
• All four types fall back to the generic DatabaseTable handler, which emits a plain ibis.table(name=..., schema=...) and silently discards parent, remote_expr, input_expr, read_kwargs, etc.
• Tests only — no fix yet. CI should show 8 failures confirming the bug.

Test plan

• 8 new tests in python/xorq/vendor/ibis/expr/test_decompile.py
  • 4 dispatch registration checks (handler is not the DatabaseTable fallback)
  • 4 output checks (decompiled code mentions cache/remote/read_parquet/udxf)
• All 8 fail on current main
• Fix commit will register handlers and make all 8 pass

Closes #1702

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 28.04878% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
python/xorq/vendor/ibis/expr/test_decompile.py	28.04%	59 Missing ⚠️

Files with missing lines	Coverage Δ
python/xorq/vendor/ibis/expr/test_decompile.py	28.04% <28.04%> (ø)

... and 192 files with indirect coverage changes

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[codspeed-hq]

Merging this PR will degrade performance by 33.89%

❌ 1 regressed benchmark
✅ 13 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_into_backend_cache	233.4 ms	353 ms	-33.89%

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_{Comparing paddymul:fix/decompile-custom-relation-handlers (65467eb) with main (fc2a75b)}