Add typed attribute converters for session, request, flash and servletContext

[codeconsole]

Summary

Adds null-safe, typed conversion methods to the session, request, flash and servletContext objects, mirroring the existing params.int('id', 1) API. Also adds a missing string/getString converter to TypeConvertingMap itself.

Integer page    = request.int('page', 1)
String  tz      = session.string('userTimeZoneId', 'America/Los_Angeles')
Boolean active  = session.boolean('active', false)
String  notice  = flash.string('notice')
int     uploads = servletContext.int('maxUploads', 10)

The full converter set is available — string, byte, char, short, int, long, double, float, boolean, list, date — each with an optional default-value overload, exactly as on params.

Motivation

Under @CompileStatic / @GrailsCompileStatic, dynamic attribute access such as session.userTimeZoneId does not compile, and the explicit form requires a cast at every call site:

String tz = (session.getAttribute('userTimeZoneId') as String) ?: 'America/Los_Angeles'

These converters give the same null-safe, typed, default-aware ergonomics as params, and they compile under static compilation. They are equally useful in dynamic code for the parsing/defaulting behaviour (just like params.int(...)).

Design

• Conversion logic is extracted into an internal helper, org.grails.util.TypeConverters, which converts a raw Object value to the target type (with an optional default). Both TypeConvertingMap's instance getX methods and the new attribute extensions delegate to it, so the logic lives in a single place and no new conversion methods are added to the public TypeConvertingMap API.
• The extensions call the converters directly on the attribute value, avoiding any per-access allocation.
• The instance getX(name, default) semantics are preserved, so params behaviour is unchanged (verified by GrailsParameterMapBindingSpec).
• The one deliberate exception is the boolean default: "is the value present" differs by holder (map containsKey vs. attribute set), which cannot be expressed from a value alone, so it stays per-site and is documented inline.
• The string converter returns the first element when the attribute is an array, mirroring the single-value semantics of the other converters; use list for all values.

Tests & docs

• Unit tests for the internal converters (incl. default-value coercion) and for getString/string.
• A Spock spec per holder (HttpSessionExtensionSpec, HttpServletRequestExtensionSpec, ServletContextExtensionSpec, FlashScopeExtensionSpec) covering conversion, null-safety, defaults, and a @CompileStatic static-resolution guard.
• GrailsParameterMapBindingSpec passes unchanged (regression guard for the core refactor).
• New "Type Conversion of Attributes" section in the controllers guide.

[matrei]

@codeconsole Can this target 8?
Decided in the weekly meeting May 27th, point 1, 7.x.x is not accepting new features.
Focus should now be on 8+.

[codeconsole]

@codeconsole Can this target 8? Decided in the weekly meeting May 27th, point 1, 7.x.x is not accepting new features. Focus should now be on 8+.

@matrei done, how about a review ;P

[codecov]

Codecov Report

❌ Patch coverage is 63.47826% with 84 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.3575%. Comparing base (41f9fb9) to head (60980ef).

Files with missing lines	Patch %	Lines
.../grails/web/servlet/ServletContextExtension.groovy	16.6667%	20 Missing ⚠️
...ache/grails/core/internal/util/TypeConverters.java	84.8214%	7 Missing and 10 partials ⚠️
...ils/web/servlet/HttpServletRequestExtension.groovy	29.1667%	16 Missing and 1 partial ⚠️
.../org/grails/web/servlet/FlashScopeExtension.groovy	30.4348%	15 Missing and 1 partial ⚠️
...org/grails/web/servlet/HttpSessionExtension.groovy	41.6667%	13 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##                8.0.x     #15715        +/-   ##
==================================================
+ Coverage     48.3483%   48.3575%   +0.0092%     
- Complexity      15104      15181        +77     
==================================================
  Files            1870       1872         +2     
  Lines           85459      85571       +112     
  Branches        14901      14899         -2     
==================================================
+ Hits            41318      41380        +62     
- Misses          37805      37851        +46     
- Partials         6336       6340         +4     

Files with missing lines	Coverage Δ
.../groovy/grails/util/AbstractTypeConvertingMap.java	81.2500% <100.0000%> (+3.2839%)	⬆️
...c/main/groovy/grails/util/TypeConvertingMap.groovy	87.5000% <100.0000%> (+1.1364%)	⬆️
...org/grails/web/servlet/HttpSessionExtension.groovy	44.8276% <41.6667%> (-15.1724%)	⬇️
.../org/grails/web/servlet/FlashScopeExtension.groovy	30.4348% <30.4348%> (ø)
...ache/grails/core/internal/util/TypeConverters.java	84.8214% <84.8214%> (ø)
...ils/web/servlet/HttpServletRequestExtension.groovy	15.1163% <29.1667%> (+5.4389%)	⬆️
.../grails/web/servlet/ServletContextExtension.groovy	15.3846% <16.6667%> (+15.3846%)	⬆️

... and 6 files with indirect coverage changes

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[codeconsole]

@sbglasius @jdaugherty @matrei annoying that I got bonus changes from other peoples work when I switched the base to 8.0.x, but I replayed everything on top of a fresh 8.0.x then force pushed this PR. Should be clean now.

[testlens-app]

✅ All tests passed ✅

⚠️ TestLens detected flakiness ⚠️

Test Summary

Check	Project/Task	Test	Runs
CI / Functional Tests (Java 21, indy=false)	:grails-test-examples-scaffolding:integrationTest	UserControllerSpec > User list	❌ ✅ 🚫 🚫

🏷️ Commit: 60980ef
▶️ Tests: 9385 executed
⚪️ Checks: 43/43 completed

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