add "Proper memory safety with GC.@preserve"

[kashish2210]

Add GC.@preserve and enhance table string operations for Julia nightly compatibility

Description

This PR addresses the Julia nightly compatibility issues of this #196 highlighted in #194 by improving string handling in table operations and ensuring proper memory management. Key improvements include the addition of GC.@preserve blocks and a new safe_table_string_convert function.

Changes

1. String Handling Improvements

Added new safe_table_string_convert function for safer string handling in table operations:
The below function was initiated in #196 I have updated a lit bit:)

function safe_table_string_convert(str::AbstractString)
    GC.@preserve str begin
        # Ensure string data remains valid during conversion
        converted = # string conversion logic
        return converted
    end
end

Implemented type traits pattern to prevent method overwriting:

abstract type VarColHandler end
struct StringVarColHandler <: VarColHandler end
struct NumericVarColHandler <: VarColHandler end
struct UnsupportedVarColHandler <: VarColHandler end

[codecov]

Codecov Report

Attention: Patch coverage is 81.36201% with 52 lines in your changes missing coverage. Please review.

Project coverage is 86.85%. Comparing base (52a53d1) to head (45e54d4).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
src/table.jl	67.52%	38 Missing ⚠️
src/fits.jl	89.24%	10 Missing ⚠️
src/header.jl	94.20%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   91.10%   86.85%   -4.25%     
==========================================
  Files           5        5              
  Lines         697      799     +102     
==========================================
+ Hits          635      694      +59     
- Misses         62      105      +43     

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[giordano]

Why is this needed? Why not using String(copy(x)) instead?

[kashish2210]

This function serves a specific purpose that String(copy(x)) doesn't handle:

• Null Termination Handling

for example

# With null terminator
data = UInt8[65, 66, 67, 0, 68, 69]  # "ABC\0DE"

# Using String(copy(x)):
String(copy(data))  # Would give "ABC\0DE" (includes null and data after it)

# Using safe_string_convert:
safe_string_convert(data)  # Gives "ABC" (stops at null)

[giordano]

Why? ENV["JULIA_CPU_THREADS"] is also the wrong way to check the number of threads for two reasons:

1. the environment variable may not be defined at all and you'd get an error in that case. To reference an env var which may not be defined you should use get(ENV, "JULIA_CPU_THREADS", <a default value here>)
2. the right way to check the number of threads is Threads.nthreads()

[kashish2210]

i waill update this thanks for ur suggestion :)

[giordano]

Why do you need to preserve s here?

[kashish2210]

this was unessary implmentaion let me clear it

[jishnub]

The issue on master has been fixed now, so this PR should not be necessary anymore.

[kashish2210]

No worries thanks for letting me know :)