[LANG-1827] Support escaped single quotes ('') in DurationFormatUtils format patterns

[sarankumarbaskar]

DurationFormatUtils.lexx() does not handle escaped single quotes, making it
impossible to include a literal apostrophe in formatted duration output.

The root cause is in the lexx() parser: when it encounters a single quote
while inside a literal block, it unconditionally ends the literal. It never
checks whether the next character is also a single quote, which should mean
"emit a literal apostrophe and continue the literal."

This behavior contradicts the quoting convention established by
java.text.SimpleDateFormat, where '' consistently produces a literal
single quote. The class Javadoc acknowledged the limitation explicitly:

"Note: It's not currently possible to include a single-quote in a format."

and a TODO in lexx() read:

// TODO: Need to handle escaping of '

This PR modifies lexx() to recognise '' as an escaped single quote — both
inside and outside quoted literal sections — matching SimpleDateFormat
behavior. The Javadoc has been updated to document the '' convention and
the TODO has been removed.

Note: This is a minor behavioral change. Existing format strings that relied
on '' as "close one literal then open another" will now produce a literal
apostrophe instead. This aligns with what most users would expect.

This issue is tracked asLANG-1827.

• Read the contribution guidelines for this project.
• Read the ASF Generative Tooling Guidance.
• Ran a successful build with mvn.
• Unit tests cover all new behavior and verify no regressions.
• Commit message follows the [LANG-XXXX] convention.

[garydgregory]

@ppkarwasz @chtompki
Do you foresee any issues with this solution aside from the obvious change in behavior?

[garydgregory]

Hello @Sarankumar18
Please see my comment.
Thank you!

[garydgregory]

Hello @Sarankumar18
Please write this new test as a @ParameterizedTest so that we can run all input/output individually and see them all fail if the main side of the patch is not applied.

[sarankumarbaskar]

@garydgregory I've made as parametrize test for testLANG1827