src: safely remove the last line from dotenv

[islandryu]

Refs: #55925

When _GLIBCXX_ASSERTIONS is enabled, passing npos to remove_prefix triggers an assertion error. Modified to avoid calling remove_prefix when npos is encountered.

#0  0x0000ffffa799e020 in __pthread_kill_implementation () from /lib64/libc.so.6
#1  0x0000ffffa794b140 [PAC] in raise () from /lib64/libc.so.6
#2  0x0000ffffa79359c8 [PAC] in abort () from /lib64/libc.so.6
#3  0x0000ffffa7cc20c8 [PAC] in std::__glibcxx_assert_fail(char const*, int, char const*, char const*) () from /lib64/libstdc++.so.6
#4  0x000000000195c634 [PAC] in std::basic_string_view<char, std::char_traits<char> >::remove_prefix (this=0xffffd0a72938, __n=18446744073709551615) at /usr/include/c++/14/string_view:297
#5  0x000000000195b98c [PAC] in node::Dotenv::ParseContent (this=0x8c3adf8 <node::per_process::dotenv_file>, input="API_KEY=\"inaaa\"") at ../../src/node_dotenv.cc:185
#6  0x000000000195be60 [PAC] in node::Dotenv::ParsePath (this=0x8c3adf8 <node::per_process::dotenv_file>, path=".env") at ../../src/node_dotenv.cc:276
#7  0x00000000018dceb4 [PAC] in node::InitializeNodeWithArgsInternal (argv=0xe11eda0, exec_argv=0xe11edb8, errors=0xe11edd0, flags=node::ProcessInitializationFlags::kNoFlags) at ../../src/node.cc:860
#8  0x00000000018dd650 [PAC] in node::InitializeOncePerProcessInternal (args=std::vector of length 2, capacity 2 = {...}, flags=node::ProcessInitializationFlags::kNoFlags) at ../../src/node.cc:1003
#9  0x00000000018de97c [PAC] in node::StartInternal (argc=2, argv=0xe11ece0) at ../../src/node.cc:1432
#10 0x00000000018ded04 [PAC] in node::Start (argc=2, argv=0xffffd0a753f8) at ../../src/node.cc:1491
#11 0x0000000003935f54 [PAC] in main (argc=2, argv=0xffffd0a753f8) at ../../src/[node_main.cc:97](http://node_main.cc:97/)

In the case of Windows, a debug build is likely to emit a warning when npos is passed to remove_prefix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.99%. Comparing base (1919560) to head (c64de0c).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #55982      +/-   ##
==========================================
- Coverage   88.00%   87.99%   -0.01%     
==========================================
  Files         653      653              
  Lines      187872   188097     +225     
  Branches    35888    35942      +54     
==========================================
+ Hits       165328   165522     +194     
- Misses      15720    15738      +18     
- Partials     6824     6837      +13     

Files with missing lines	Coverage Δ
src/node_dotenv.cc	92.77% <100.00%> (+0.24%)	⬆️

... and 30 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63684/