inspector: report an error when DOM storage is unavailable

[zeexzeex]

getDOMStorageItems() falls back to reading the live Storage object when its cached map is empty. When getWebStorage() cannot produce one, the fallback was skipped and the command still answered with Success and an empty entries array. A frontend could not tell an empty store from a store that cannot be read.

getWebStorage() returns nothing whenever globalThis.localStorage is missing or is not an object, which is the case for any process started without --localstorage-file.

This returns a ServerError in that case, matching how the other failure paths in the same command report problems.

Note that this changes what a frontend sees for a process started without --localstorage-file, which is the common case. sessionStorage is unaffected: it is always backed by an in-memory store, so it stays readable and keeps answering with an empty list.

Fixes: #65895

[nodejs-github-bot]

Review requested:

• @nodejs/inspector

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[codecov]

Codecov Report

❌ Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.22%. Comparing base (bd9d82d) to head (a1f59f6).
⚠️ Report is 73 commits behind head on main.

Files with missing lines	Patch %	Lines
src/inspector/storage_agent.cc	54.54%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65973      +/-   ##
==========================================
+ Coverage   90.16%   90.22%   +0.05%     
==========================================
  Files         771      785      +14     
  Lines      265485   269356    +3871     
  Branches    50466    51519    +1053     
==========================================
+ Hits       239381   243021    +3640     
+ Misses      17043    16857     -186     
- Partials     9061     9478     +417     

Files with missing lines	Coverage Δ
src/inspector/dom_storage_agent.cc	88.26% <100.00%> (+1.94%)	⬆️
src/inspector/storage_agent.cc	79.16% <54.54%> (-20.84%)	⬇️

... and 128 files with indirect coverage changes

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[zeexzeex]

The test I added called Storage.getStorageKey without --localstorage-file, which aborted the process on Linux CI: localstorage_file is empty there and std::filesystem::absolute() throws on an empty path under libstdc++, while libc++ returns the current directory. That is why it passed locally on macOS.

This adds a fix for that in storage_agent.cc, so the PR now touches two files it did not before. Flagging it since you have already approved.